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IMAGE  EVALUATION 
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Photographic 

Sciences 
Corporation 


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33  WIST  MAIN  STMIT 

WItfTH.N.Y.  I4SM 

(71*)  •72-4S03 


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CIHM/ICMH 

Microfiche 

Series. 


CBHM/ICIVIH 
Collection  de 
microfiches. 


Canadian  Institu'a  for  Historical  Microreproductions  /Institut  canadier  da  microreproductiont  historiquas 


Technical  and  Bibliographic  Notes/Notas  techniques  et  bibliographiques 


The  Institute  has  attempted  to  obtain  the  best 
original  copy  available  for  filming.  Features  of  this 
copy  which  may  be  bibliographically  unique, 
which  may  alter  any  of  the  images  in  the 
reproduction,  or  which  may  significantly  change 
the  usual  method  of  filming,  are  checked  below. 


L'Institut  a  microfilm^  le  meiileur  exemp!aire 
qu'il  lui  f  6t6  possible  de  se  procurer.  Les  details 
de  cet  exempiaire  qui  sont  peut-dtre  uniques  du 
point  de  vue  bibliographique,  qui  peuvent  modifier 
une  image  reproduite,  ou  qui  peuvvrnt  exiger  una 
modification  dans  la  mdthode  norma!e  de  filmage 
sont  indiquds  ci-dessous. 


D 


D 


Coloured  covers/ 
Couverture  de  couleur 


I      I    Covers  damaged/ 


Couverture  endommagde 


Covers  restored  and/or  laminated/ 
Couverture  restaurie  et/ou  pelliculAe 


I      I    Covei  title  missing/ 


n 
□ 


Le  titre  de  couverture  manque 

Coloured  maps/ 

Cartes  g^ographiques  en  couleur 


Coloured  ink  (i.e.  other  than  blue  or  black)/ 
Encre  d9  coubur  (i.e.  autre  que  bleue  ou  noire) 


Coloured  plates  and/or  illustrations/ 
Planches  et/ou  illustrations  en  couleur 


n 


Bound  with  other  material/ 
Reti(i  avec  d'autres  documents 

Tight  binding  may  cause  shadows  or  distortion 
along  interior  margin/ 

La  reliure  serr^e  peut  causer  de  I'ombre  ou  de  la 
distortion  le  long  de  la  marge  intirieure 

Blank  leaves  added  during  'dstoratlon  may 
appear  within  the  text.  Whenever  possible,  these 
have  been  omitted  from  filming/ 
II  se  peut  que  certaines  pages  blanches  ajouties 
lors  d'une  restauration  apparaissent  dans  le  texte. 
mtaia,  lorsque  cela  Atcit  possible,  ces  pages  n'ont 
pas  hxk  filmies. 


□    Coloured  pages/ 
Pages  de  couleur 

r~rKPages  damaged/ 
L.IlI    Pages  endommagies 

r~~|    Pages  restored  and/or  laminated/ 


Pages  restaurdes  at/ou  pelticuldes 

Pages  discoloured,  stained  or  fox^id/ 
Pages  ddcolordes,  (achetias  ou  piqu^es 


□    Pages  detached/ 
Pages  d6tac!i6os 

I     T^  Showthrough/ 
L^fd    Transparence 

□    Quality  of  print  varies/ 
Qualiti  in^gale  de  I'impression 

□    Includes  supplementary  material/ 
Comprend  du  materiel  supplementaire 

□    Only  edition  Available/ 
Seule  Mitiuri  disponible 


D 


Pages  wholi/  or  partially  obscured  by  errata 
slips,  tisrue:i,  etc.,  i^ave  been  refilmed  to 
ensure  the  best  possible  irrage/ 
Les  pages  totalement  ou  partiellement 
obscurcies  par  un  feuillst  d'errata,  une  pelure, 
etc.,  ont  ^tA  film493  h  nouveau  de  fa^on  h 
obtenir  la  meilleure  image  possible. 


D 


Additionel  comments:/ 
Commentaires  supplAmentalres; 


This  item  is  filmed  at  the  reduction  ratio  checked  below/ 

Ce  document  est  film*  au  taux  de  riduction  indiqut  ci-dessous. 


10X 


14X 


18X 


?2X 


2iX 


30X 


y 


12X 


16X 


20X 


a4X 


28X 


32X 


The  copy  filmed  here  has  b«en  reproduced  thanks 
to  the  generosity  of: 

Dana  Porter  Arts  Library 
University  of  Waterloo 


L'exemplaire  film6  f ut  reproduit  grflce  A  la 
gAn6rosit6  de: 

Dana  Porter  Arts  Library 
University  of  Waterloo 


The  images  appearing  here  are  the  bsst  quality 
possible  considering  tho  condition  and  legibility 
of  the  original  copy  and  in  keeping  with  the 
filming  contract  specifications. 


Las  images  suivantes  ont  6ti  reproduites  avec  ie 
plus  grand  soin.  compte  tenu  de  ia  condition  at 
de  l&  nettetA  de  l'exemplaire  fiimt.  at  en 
conformity  avec  les  conditions  du  contrat  de 
filmage. 


Original  copies  in  printed  paper  covers  are  filmed 
beginning  with  the  front  cover  and  ending  on 
the  last  page  with  a  printed  or  illust/ated  impres- 
sion, or  the  back  cover  when  appropriate.  All 
other  original  copies  are  filmed  beginning  on  the 
first  page  with  a  printed  or  illustrated  impres- 
sion, and  ending  on  the  last  page  with  a  printed 
or  illustrated  impression. 


Les  exemplaires  originaux  dont  la  oouverture  en 
papier  est  imprimAe  sont  filmte  en  commen^ant 
par  Ie  premier  plat  at  en  terminant  soit  par  la 
derniire  page  qui  comporte  une  empreinte 
d'impression  ou  d'iilustration.  soit  par  Ie  second 
plat,  selon  Ie  cas.  Tous  les  autres  exemplaires 
originaux  sont  fllmte  en  commandant  par  la 
premiere  pago  qui  comporte  une  empreinte 
d'impression  ou  d'iliustration  at  en  terminant  par 
la  derniAre  page  qui  v<:omporte  une  telle 
empreinto. 


The  last  recordiad  frame  on  each  microfiche 
sh.ili  contain  the  symbol  —^  (meaning  "CON- 
TINUED"), or  the  symbol  V  (meaning  "END"), 
whichever  appiios. 


Un  des  symboles  suivants  apparattra  sur  la 
derni6re  image  de  cheque  microfiche,  selon  Ie 
cas:  ie  symbole  — ^  signifie  "A  SUIVRE",  Ie 
symbols  V  signifie  "FIN". 


Mnps,  plates,  charts,  etc.,  may  be  filmed  at 
different  reduction  ratios.  Those  too  large  t j  be 
entirely  included  i.i  one  exposure  are  filmed 
beginning  in  the  upper  left  hand  corner,  left  to 
right  and  top  to  bottom,  as  many  frames  as 
required.  The  following  diagrams  illustrate  the 
method: 


Les  cartes,  planches,  tableaux,  etc..  peuvent  Stre 
filmto  A  des  taux  de  r6duction  diff Arents. 
Lorsque  Ie  document  est  trop  grand  pour  Atre 
reproduit  en  uii  seul  clichi,  il  est  fiimi  A  partir 
de  I'angle  sup6rieur  gauche,  de  gauche  A  droite, 
et  de  haut  en  has,  en  prenan'*:  Ie  nombre 
d'imefies  nicessaire.  Les  diagrammes  suHvants 
illustrent  ia  m6thode. 


1 

2 

3 

6 


xri 


professor 


Ubc  Science  Series 

EDITED    nv 

Iprofcsecr  J.  Obctieen  Cattell,  /B.H.,  pb.H). 

AND 

f.  £.  Sc^^ar^,  /B.B.,  f  .1R.S. 


RIVERS  OF  NORTH 
AMERICA 


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RIVERS  OF   NORTH 
AMERICA/ 


A  READING  LESSON  FOR  STUDENTS 
01'  GEOGRAPHY  AND  GEOLOGY 


L^' 


HY 

//  // 

ISRAEL  C.   RUSSULL 

PkOFFSSOK    OF   r.EOLOGV    IN    THE    UNI'.  RSSr-V    ,)F   MICHIGAN 

AUTHOK    (.F    "lakes    OF    NORTH    AMHkICA,"    "  GLACiri  <"    -l    NORTH    AMFkICA," 

"volcanoes   of    north    AMERICA   '■    KT>.. 


NKW    VOKK 

G.  P.  PUTNAM'S   SONS 

LONDON 

JOHN  MURRAY 
1898 


,  5 


;i 


=SK.«;a 


Copyright,  1898 

BV 

G.  P.  PUTNAM'S  SONS 


Zb'.  1«nlclicrlttocl»cr  pteet,  "ttew  Beth 


Every  nver  appears  to  consist  of  a  main  trunk,  fe.l  from  a  variety  of 
Whes  each  running  in  a  valley  proportioned  to  its  size,  and  all  of  them 
^U,er  form,ng  a  system  of  valleys,  communicating  with  one  another,  and 
having  such  a  n.ce  adjustment  of  their  declivities,  that  none  of  them  join  the 

wl".7^"     '.  T        ""  ^"^  ''«'  '''  ^°°  ^°"-  ^  '^-'  ■'  ^  circumstancl  vvh  cl 
woold  be  .nfinuely  improbable  if  each  of  these  valleys  were  not  the  work  o 

:  i*r  tariA  :  by  John  Play  fair.     Edinhurgh,  1S02.    /.  102. 


3 


f) 


HI^^^HI^^BC 

1 

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waters 

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also,  th 

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though 

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:/ 


TO  THE  READER 


EVERY  person  is  familiar  with  the  beiting  of  the  rain 
upon  the  surface  of  the  land,  and  t'le  gathering  of  the 
waters  that  fall  into  rills,  rivulets,  ar.  y  brooks  which  fre- 
quently unite  to  form  larger  rivers.  Everyone  is  aware, 
also,  that  streams  are  turbid  aftc  ,neavy  rains.  But,  al- 
though these  facts  have  been  known  to  us  from  childhood, 
yet  comparatively  few  people  have  thought  out  the  chain  of 
events  of  which  they  form  a  part,  or  recognised  the  results 
toward  which  they  lead. 

Standing  by  the  side  of  a  river,  we  see  its  waters  flowing 
continually  in  one  direction  and  in  many  instances  bearing 
along  a  load  of  sediment.     We  know  that  the  mud  which 
discolours  the  waters  was  derived  from  the  lands  bordering 
the  stream  and  is  journeying  to  the  sea.     So  far  as  we  can 
ordinarily   discern,  there   is  no  compensation  for  this  re- 
moval.    Evidently,   if  the  process  goes  on  without  being 
counteracted  by  other  agencies,  all  of  the  material  forming 
land  areas  will  in  time  be  removed,  and  the  hills  and  even 
I  the  grandest  mountains  will  be  degraded  to  the  level  of  the 
[sea.    We  know  of  no  reason  why  this  process  of  soil  removal 
may  not  have  been  in  operation  since  rain  first  fell  on  land, 
[or  why  it  may  not  continue  so  long  as  continents  and  islands 
lexist.     As  the  land  has  not  been  reduced  to  sea-level,  one 


'i 


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I 

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VI 


TO   THE  READER 


°^  ^''Vro  conclusions  seems  evident:  either  the  time  that  has 
elapsev^  gj^ce  the  process  began  has  not  been  sufficiently 
long  to  .^^jj^g  ^^^^^  ^hg  final  result,  or  else  there  is  some 
compens.  ^^.^^  process  by  which  land  areas  are  renewed. 

A  persoi.^  ^^^  wanders  along  a  river  bank  with  these  and 
kindred  tho^.^gj^^^  -^^  ^^^^  naturally  seeks  for  evidences  of 
the  work  the      .^^^.^  ^^^^  accomplished  and  endeavours  to 
learn  how  it  has     ^^^^^  carried  on.     One  observes  the  river 
flowing  through  a^  narrow,  steep-sided  trench,   or  perhaps 
meandering  in  broat  ^^  graceful  curves  over  the  bottom  of  a 
wide,  fruitful  valley.        j^  all  directions  the  view  is  limited  by 
hills  or  steeply  ascendi.*,^^  slopes.     On  gaining  a  command- 
ing station  on  a  hilltop,  tn^     broader   prospect,  including 
both  hills  and  valleys,  very  likely  win  revcai  tne  tact  that 
the  hills,  ridges,  and  more  or  less  isolated  peaks  rise  to  the 
same  general  height,  and  appear  as  a  level  plain  when  the 
eye,  nearly  on  a  level  with  their  summit,  ranges  over  them; 
and  sunken  in  this  plain  is  the  river  valley.     If  the  river  has 
been  engaged  for  ages  in  carrying  away  the  material  of  the 
land  in  the  way  it  is  now  doing,  the  valley  must  represent 
a  part  or  the  whole  of  the  work  done.     The  thought  that 
the  river  is  older  than  the  valley,  and  that  the  valley  has 
been  excavated  by  the  river,  comes  to  one  like  a  revelation. 
In  fancy  we  see  the  valley  filled  with  rocks  like  those  in  the 
bordering  hills  and  the  plain  restored.     Evidently  the  sur- 
face of  the  region,  less  diversified  than  now,  must  have  been 
a  plain  or  plateau  before  the  stream  excavated  its  valley. 
When  this  idea  has  taken  root  in  the  mind,  our  powers  of 
observation  are  stimulated  and  our  faith  strengthened  in 
what  has  been  termed  the  scientific  use  of  the  imagination. 


TO    THE  READER 


VU 


As  our  vision  ranges  over  valleys  and  hills,  the  fact  is  recog- 
nised that  the  neighbouring  mountains  are  but  uplands  of 
larger  size,  separated  one  from  another  by  gorges  and  val- 
leys, in  each  of  which  a  stream  is  flowing,  and  we  are 
startled  by  the  vividness  of  the  pictures  that  crowd  them- 
selves on  our  fancy,  each  portraying  a  different  stage  in  the 
development  of  a  landscape  which  had  previously  charmed 
us  simply  by  its  assemblage  of  attractive  forms  and  its 
harmonious  blendings  of  colour.  • 

The  mental  pictures  of  the  loiterer  by  a  stream-side  or 
the  dreamer  on  a  hilltop  are  not  confined  to  the  region  im- 
mediately before  him.  What  is  true  of  the  stream  at  his 
feet  must  also  pertain  in  general  to  other  streams  in  what- 
ever clime.  Again,  a  curtain  is  lifted  and  he  sees  that  every 
stream  on  the  earth's  surface  is  engaged  in  changing  the  as- 
pect of  the  land.  Valleys  have  been  excavated  on  every 
continent  and  island.  Every  mountain  has  been  sculptured. 
Changes  due  to  running  water  are  everywhere  in  progress. 
If  these  conclusions  are  well  founded,  it  is  evident  that 
valleys,  and  mountains  are  but  transient  forms  in  a  long 
process  of  topographical  development,  and  the  history  of 
past  changes  should  find  expression  in  the  relief  of  the 
land. 

The  leading  idea  which  absorbs  the  attention  when  a 
living  interest  is  once  awakened  in  the  meaning  of  the  many 
and  diversified  features  of  the  earth's  surface,  is  that  they 
are  not  fixed  and  changeless  forms,  but  have  undergone 
many  orderly  modifications  in  the  past  and  will  continue  to 
change  under  the  action  of  definite  laws  in  the  future.  It 
is  not  the  shape  of  the  earth  as  it  exists  to-day,  the  present 


r 


li 


i 


v»u 


TO    THE  READER 


distribution  of  land  and  water  on  its  surface,  or  the  relief  of 
the  land,  or  of  the  floor  of  the  sea,  but  the  changes  that  each 
of  these  conditions  has  passed  through  in  order  to  reach 
its  present  state,  and  the  modifications  stil!  in  progress, 
which  claim  the  greatest  share  of  the  geographer's  attention. 
Evolution  is  the  leading  theme  of  the  inanimate  as  well  as 
of  the  animate  world.  The  features  of  the  earth's  surface, 
from  the  continents  and  oceans  to  the  smallest  islands  and 
tiniest  rills,  all  have  what  may  be  termed  their  life  histories. 
It  is  the  recognition  of  this  fact  that  has  given  new  interest 
and  imparted  a  fresh  >mpetus  to  geographical  study. 

When  (Mice  the  idea  is  grasped  that  each  and  every  one  of 
the  elements  in  a  landscape  has  a  history  which  can  be  read, 
and  that  the  end  is  not  yet,  but  still  other  transformations 
are  to  come  an  insatiable  desire  is  awakened  for  more 
knowledge  concerning  especially  the  work  of  the  streams  to 
which  so  many  of  the  changes  that  have  been  made  on  the 
earth's  surface  are  due.  What  laws  do  the  streams  obey  ? 
What  conditions  modify  their  normal  behaviour  ?  What 
are  the  various  stages  in  the  transformations  they  are  making 
everywhere  about  us  ?  Are  there  other  forces  in  action, 
tending  to  counteract  their  dei'tructive  work  ?  Are  the 
conditions  to  which  man  has  become  adjusted  to  pass  away  ? 
What  changes  are  to  come  ?  Is  the  re-modelling  of  thj 
laud  to  be  continued  for  ever,  or  will  there  be  a  final  condi- 
tion beyond  which  the  expression  of  the  face  of  Nature  will 
become  sphinx-like  and  unchanging  ?  These  and  many 
niore  queries  crowd  themselves  on  the  mind  when  once  an 
interest  in  the  daily  scenes  about  us  is  awakened.  Many, 
but  by  no  mean,  all,  of  the  questions  which  we  wish  to  ask 


TO    THE  READER 


&e 


of  the  mountains  and  streams  can  be  satisfactorily  answered 
at  the  present  day. 

It  IS  with  the  hope  of  assisting  the  reader  both  in  ques- 
tioning the  streams  and  in  understanding  their  answers,  and 
at  the  same  time  creating  a  desire  for  more  light  on  other 
and  related  chapters  of  the  earth's  history,  that  the  book 
before  you  was  written. 

The  r  dy  of  the  earth's  surface  should  be  of  especial  in- 
terest to  American  students  not  only  because  of  the  mag- 
nificent and  varied  scenery  of  our  native  land,  but  for  the 
reason  that  new  life  and  vividness  have  been  given  the  sub- 
ject by  the  labours  of  men  who  are  stili  among  us. 

The  marked  advances  made  during  the  present  century 
both  in  the  study  of  the  ancient  life  on  the  earth  and  of  the 
surface  changes  still  in  progress,  show  the  influence  of  en- 
vironment. The  Geological  Survey  of  the  State  of  New 
York  gave  a  marked  impetus  to  the  study  of  the  inverteorate 
life  of  distant  ages,  largely  for  the  reason  that  the  rocks  of 
the  New  York  series  are  rich  in  such  relics.  When  geolo- 
gists visited  the  central  and  western  portions  of  the  United 
States,  the  sediments  of  ancient  lakes,  rich  in  vertebrate 
fossils,  were  discovered.  The  veritable  menagerie  of  mar- 
vellous birds,  reptiles,  and  mammals  that  has  been  made  to 
appear  from  these  cemeteries  is  more  varied  than  the  most 
fantastic  dreams  of  fable.  We  feel  that  the  uncouth  pro- 
cession has  only  begun  to  pass  in  review,  but  are  at  a  loss 
to  ima^Mne  what  as  yet  unknown  commingling  of  fish,  rep- 
tile, bird,  and  mammal  in  one  and  the  same  individual  can 
possibly  be  found.  When  investigators  of  surface  geology 
and  geography  made  their  bold  explorations  into  the  vast 


l! 


TO    THE  READER 


arid  region  of  the  suuth-west,  they  discovered  a  land  of 
wonders,  where  the  mask  of  vegetation  which  conceals  so 
many  countries  is  absent,  and  the  features  of  the  naked  land 
are  fully  revealed  beneath  a  cloudless  sky.  The  facts  in 
the  earths  history  which  there  impress  themselves  most 
forcibly  on  the  beholder  are  such  as  have  resulted  from  the 
action  of  streams  and  of  atmospheric  agencies.  It  was  in 
this  arid  region  of  strong  lelief  that  a  revival  of  interest  in 
the  surface  forms  of  the  earth  was  engendered.  The  seeds 
of  what  is  practically  a  new  science, — physiography, — 
gathered  in  this  desert  land  by  J.  S.  Newberry,  J.  W. 
Powell,  G.  K.  Gilbert,  C.  E.  Button,  and  others,  when 
carried  to  other  regions  bore  abundant  fruit.  It  was  found 
that  the  surface  of  the  land,  when  once  suggestion  had  en- 
abled men  to  see  topographic  forms  and  interpret  their 
meaning,  is  a  manuscript  on  which  v/onderful  events  are 
recorded. 

A  younger  generation  of  active  workers  has  ext'^nded  the 
study  of  the  earth's  surface,  so  greatly  stimulated  by  the 
pioneer  explorers  just  named,  and  has  read  for  us  the  his- 
tories of  valleys,  plains,  hills,  and  mountains  throughout  the 
length  and  breadth  of  the  land.  Of  these  younger  investi- 
gators we  are  indebted  to  none  so  much  as  to  \V.  M.  Davis, 
Professor  of  Physical  Geography  at  Harvard,  who  has  made 
New  England,  New  Jersey,  and  Pennsylvania  classic  ground 
to  all  future  students  of  physiography.  Gilbert  has  ex 
tended  his  studies  to  the  basins  of  the  Laurentian  lakes  and 
other  regions.  The  writings  of  J.  C.  Branner,  M.  R.  Camp- 
bell, T.  C.  Chamberlin,  N.  H.  Darton,  J.  S.  Diller,  C.  W. 
Hayes,  Arthur  Keith,  W  J  McGee,   R.  D.  Salisbury,   K. 


TO    THE  HEADER 


S.  Tarr,  Bailey  Willis,  and  others,  have  greatly  enlarged  our 
knowledge  of  the  laws  governing  streams,  and  of  the  origin 
of  topographic  forms. 

The  publications  of  the  United  States  Geological  Survey 
and  of  the  earlier  national  surveys  of  which  it  is  a  continua- 
tion, the  Geological  and  Natural  History  Survey  of  Canada, 
various  State  surveys,  the  Geological  Society  of  America,  and 
the  National  Geographic  Society,  together  with  the  Journal 
of  Geology ,  the  American  Journal  of  Science,  etc.,  are  the 
sources  of  published  information  drawn  on  most  largely  in 
the  preparation  of  this  book. 

In  the  following  chapters  many  references  are  given  to 
the  writings  of  the  distinguished  investigators  named  above, 
but  all  of  the  help  derived  from  them  while  writing  this 
book  can  scarcely  be  acknowledged.  Much  assistance  has 
also  been  derived  from  conversation  and  correspondence 
with  my  colleagues,  and,  although  fully  appreciated,  the 
portion  derived  from  each  one  is  scarcely  known  even  to 
myself.  My  part  i.i  presenting  this  book  is  largely  that  of 
a  guide  who  points  out  the  routes  others  have  traversed. 
My  reward  will  be  ample  if  even  a  few  students  follow  the 
paths  indicated  and  are  led  to  explore  their  many  as  yet 
unknown  branches. 


r 


t 


I 


e 


Israel  C.  Russell. 


University  of  Michigan, 
December  to,  i8j7. 


CONTENTS 


To  THE  Reader     .        .        .        *        ,        ; 


I  AGE 
V 


CHAPTER   I 

The  DlSINTKGRATION  AM)  DECAY  OF  RoCKS 

Mechanical  Disintegration — Chemical  Disintegration  or  Rock  Decay 
— Removal  and  Renewal  of  Surface  Debris. 


,  CHAPTER  11 
Laws  Governing  the  Streams 12 

How  Streams  Obtain  their  Loads — Transportation— Debris  Carried 
by  Ice  —  Corrasion  —  Pot- Holes — Lateral  Corrasion — Meandering 
Streams — Other  Curves — Deflection  of  Streams  Owing  to  the  Earth's 
Rotation — Questioning  the  Rivers — Erosion — Baselevel  of  Erosion — 
Peneplains — Influence  of  Vegetation  on  Erosion. 

CHAPTER  III 

Influence  of  Inequalities  in  the  Hardness  of  Rocks  on  River- 

SIDE  Scenery      .       ,       ♦       «       , 5a 

Waterfalls — The  Migration  of  Wa'erfalls — Bluffs  Bordering  Aged 
Streams. 


CHAPTER  IV 

Material  Carried  by  Streams  in  Suspension  and  in  Solution 

The  Visible  Loads  of  Streams — Bottom  Load — Measures  of  M.nterial 
in  Suspension — The  Invisible  Loads  of  Streams — R.ite  of  Land 
Degradation — Mechanical  Degradation — Chemical  Degradation — 
Rate  of  Both  Mechanical  and  Chemical  Degradation — Underground 
Streams.  r 


67 


V 


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I 

r  . 


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) 


\ 


w^ 


•BW" 


XIV 


CONTENTS 


CHAPTER  V 


Stream  Deposits 


Alluvial  Coiids — Talus  Slopes — Flood-Plains — Natural  Levees — 
Deltas :  Deltas  of  High-Grade  Streams — Deltas  of  Low-Grade 
Streams — Effects  of  Changes  in  the  Elevation  of  the  Land  on 
the  Growth  of  Deltas — Variations  in  Normal  Stream  Deposition — 
Influence  of  Elevation  and  Depression  of  the  Lard  on  Stream 
Deposition — Influence  of  Variations  in  Load  on  Stream  Deposition 
— Influence  of  Changes  of  Climate  on  Stream  Deposition— The 
General  Process  of  Stream  Corrasion  and  Deposition — Profiles  of 
Streams — The  Longitudinal  Profile— Cross-Profiles. 


PACE          J 

■ 

97      1 

BSOME  OF 

1         I^ra 

I         ^^''i 

■         of  th 

■         Rive 

■        Rive 

B        Nev2 

I        Nort 

■        "Gr< 

■        Niag 

CHAPTER  VI 


Stream  Terraces 

Origin  of  Terraces  during  the  Process  of  Normal  Stream  Develop- 
ment— Terraces  Due  to  Climatic  Changes — Terraces  Due  to  Eleva- 
tion of  the  Land — Bottom  Terraces — Delta  Terraces  and  Current 
Terraces — Glacial  Terraces — Relative  Age  of  Terraces — Other  Ter- 
races— General  Distribution  of  Stream  Terraces. 


152 


CHAPTER  VII 


Stream  Development 184 

Consequent  Streams — Subsequent  Streams — Ideal  Illustration  of 
Stream  Adjustment  and  Development — Examples  of  Stream  Develop- 
ment and  Adjustment  in  the  Appalachian  Mountains — Influence  of 
Folds  in  the  Rocks  on  Stream  Adjustment — Water-Gaps  and  Wind- 
Gaps — Stream  Conquest — Ancient  Peneplains — Synclinal  Mountains 
and  Anticlinal  Valleys — Effects  of  Elevation  and  Subsidence  on 
Stream  Development — Some  of  the  Effects  of  Elevation — Some  of 
the  Effects  of  Subsidence — Some  of  the  Influences  of  Volcanic 
Agencies  on  Stream  Development— Some  of  the  Modifications  in 
Stream  Development  Due  to  Climatic  Changes — Variations  in  Pre- 
cipitation— Variations  in  Temperature — Fluctuations  of  I~treams — 
Some  of  the  Influences  of  Glaciers  on  Stream  Development — Some 
of  the  Influences  of  Vegetation  on  Stream  Development — Driftwood 
— Superimposed  Streams — Migration  of  Divides. 


CONTENTS 


XV 


CHAPTER  VIII 

SOME  OF  THE  CHARACTERISTICS  OF  AMERICAN  RiVERS  .... 
Drainage  Slopes :  Atlantic,  St.  Lawrence,  Hudson  Bay,  Arctic, 
Bering,  Pacific,  Great  Basin,  Gulf,  and  Caribbean — Leading  Features 
of  the  Several  Drainage  Slopes — New  England  Rivers — A  Drowned 
River— Appalachian  Rivers— Rivers  of  Glaciated  Lands— Southern 
Rivers— Alluvial  Rivers— The  Mississippi— Canyor.  Rivers— Sierra 
Nevada  Rivers — "  Where  Rolls  the  Oregon  "—Rivers  of  the  Far 
North-West— Glacier-Born  Rivers — Arctic  Rivers— Rivers  of  the 
"Great  Lone  L?.nd" — Rivers  Flowing  to  Fresh- Water  Seas — 
Niagara — Retrospect. 


PACB 

254 


CHAPTER  IX 

"he  Life  History  of  a  River  . 


.     301 


index 


321 


«  I 


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I. 

A  Po 

2, 

A  Yo 

3. 

Prof 

4- 

Cros. 

5- 

Map 

6. 

RADli 

7- 

Long 

8. 

Succi 

9- 

iDEAl 

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1. 

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2. 

Allua 

3- 

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♦• 

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ABLE  A. 

ILLUSTRATIONS  IN  THE  TEXT 


IGURE  PACK 

I.      A  POT-HOLE  BEING  SCOURED  OUT  BY  A  ;     'REAM        .  >,  ;  .         33 

A  Young  Stream  NEAR  Ithaca,  New  York         ...        .        .       55 

Profile  OF  Niagara  Falls  ...        .        .        ...       60 

Cross-Profile  OF  A  Floodplain  .        .        .        .        .        .        .118 

Map  OF  the  Lower  Mississippi  showing  Crevasses  .        .        .119 

Radial  Section  of  a  Delta 126 

Longitudinal  Profile  OF  A  Young  Stream  4,  A  .  147 
Successive  Changes  in  the  Profile  of  a  Divide  .  .  .148 
Ideal  Profile  OF  A  Divide  .  .  ♦  ;.  >  -.  .  .  149 
Cross-Prof'le  OF  A  Terraced  Valley        ,     ^  ,    ,    .        .        .152 

1.  Alluvial  Terraces 156 

2.  Alluvial  Terraces 157 

Cross-Section  of  a  Valley  with  Terraces  in  Solid  Kock  .  165 
Cross-Section  OF  A  Current-built  Terrace  :  >  *  .  ,  168 
Section  OF  tilted  Peneplain       .        .        .        .        .        .        .     186 

Sketch-Map,  showing  Young  Streams 187 

Sketch-Map,  ILLUSTRATING  Stream  Development  .  ♦,  ,  189 
Sketch-Map,  showing  Mature  Streams     ,        .        .        ,/:'■',     190 

anticlinal  AND  Synclinal  .        .       ,. 198 

Map  illustrating  River  Piracy 200 

Section  through  Lookout  Mountain,  etc.,  Alabama  .  .  212 
Map  OF  Chesapeake  Bay  ....  .  »  .  .  219 
Cross-Profile  OF  Colorado  Canyon  ...        .        .        .     272 

BLE  A.    Analysis  of  American  River-Waters      .        .       facing      78 


2. 

3- 

[4- 

5- 

:6. 

Is. 

|9- 
ko. 


3- 

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15 
16. 


xvii 


1 


FULL-PAGE  ILLUSTRATIONS 


PLATE 
I. 

II. 
III. 

IV. 

V. 
VI. 

VII. 
VIII. 

IX. 
X. 

XI. 
XII. 


tvii. 


FACI.NG  PAGE 

(I.  Marion  River,  New  York.    d.  Ingall's  Ceeek,  Wash- 
ington           12 

Views  on  the  Yukon,  Alaska      ^  '     *        .        ,  ;      v       .  24 

a.  Ray   Brook,    Adirondacks,   New   York.    i.  Moccasin 

Bend,  Tennessee  River 38 

a.  Fall    on    Black  Creek,   near  Gadspen,  Alabama.    //. 

Echo  River,  Mammoth  Cave,  Kentucky       ...  60 

Map  OF  THE  Delta  of  THE  Mississippi    .        .        .        ,        .  98 

a.  Sketch  of  Alluvial  Cones,     d.  Indian   Creek,  Cali-  • 

fornia       . ,    /  .  102 

a.  Big  Goose  River,  Wyoming,    d.  New  River,  Tennessee,  108 
(7.  Terraces  on  Fraskr  River,  British  Columbia.     />.  Ter- 
races in  Connecticut  Valley         .        .        .        .        .154 

Map  of  the  Northern  Appalachians    .        ,  '     .        .  196 

Map  of  Western    Portion    of    the   Anthracite    Basin, 

Pennsylvania   .        .        .        ,        .        .        .«      .        .  204 

Map  illustrating  Stream  Adjustment        .        .  .210 

a.  Beaver  Dam,  Wyoming.    />.  Dam  of  Drift-wood,  West 

Fork  ok  Teanaway  River,  Washington        .        .        .  238 

Map  of  a  Portion  of  the  Catskill  Mountains,  New  York,  250 

Map  of  North  America  showing  Drainage  Slopes     ,       ',  256 

a.  Columbia  River,     b.  Hudson  River         ^        .        .        .  262 

a.    An    aggraded   Valley    near    Fort    Wingate,    New  . 
Mexico.    S.    Shenandoah  Peneplain,  near  Harper's 

Ferry,  West  Virginia 268 

Canyon  of  THE  Colorado       »        .        .        .        ,  v  r«'      i.  274 


,xix 


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im 


RIVERS  OF  NORTH  AMERICA 


CHAPTER  1 


THE  DISINTEGRA  TION  AND  DEC  A  V  OF  ROCKS 


THE  study  of  rivers,  from  the  point  of  view  of  the  geo- 
grapher, necessitates  the  consideration  of  the  nature 
and  origin  of  many  topographic  forms;  the  reason  being 
that  streams  are  among  the  most  important  agencies  which 
give  form  and  expression  to  the  surface  of  the  land.     The 
[study  of  streams,  therefore,  involves,  to  a  great  extent,  the 
:onsideration  of  the  origin  of  hills  and  mountains,  plains 
md  valleys,  and  the  changes  they  pass  through. 
One  of  the  principal  tasks  performed  by  streams  is  the 
loving  of  rock  fragments  and  their  transportation  to  the 
sea.     Another  function  of   streams  is  the   deepening  and 
ndening  of  their  channels  and  valleys.     These  propositions 
nil  be  demonstrated  later.     It  will  be  shown,  also,  that 
blear  water  has  but  little  power  to  wear  away  the  rocks 
^ver  which  it  flows.     In  order  to  do  this,  the  flowing  water 
lust  be  charged  with  hard  particles  or  rock  fragments  of 
reater  or  less  size.     That  is,  the  streams  must  be  supplied 
Hth  tools  with  which  to  excavate.     It  is  now  well  under- 


•■hi 


RIVERS  OF  NORTH  AMERICA 


stco-i  that  the  tools  used  by  streams  in  abrading  the  rocks 
are  mainly  silt,  sand,  gravel,  and  stones,  which  are  carried 
in  sui:pension  or  rolled  and  pushed  along  the  bottom.  One 
of  the  primary  questions,  therefore,  in  order  to  understand 
how  streams  are  enabled  to  remove  material  from  the  land, 
and,  in  so  doing,  to  deepen  and  broader  their  valleys,  is: 
How  are  the  rocks  broken  or  otherwise  prepared  for  stream 
transportation  ? 

The  soil  which  nearly  everywhere  forms  the  surface  of  the 
land  is  composed  mainly  of  disintegrated  rock.  This  loose 
surface  layer  of  more  or  less  comminuted  and  decayed 
material,  much  of  it,  however,  far  too  coarse  to  bj  termed 
soil,  is  the  storehouse  from  which  the  streams  derive  the 
principal  part  of  their  loads. 

The  study  of  the  agencies  at  work  in  breaking  and  other- 
wise disintegrating  the  earth's  crust  has  shown  that  theyj 
may  be  classified  \n  two  groups:  ist,  those  acting  mechani 
cally;  and  2d,  those  whose  influence  is  principally  chemical,  | 
Although   the   various   agencies  in  these   two   groups  co- 
operate and  are  frequently  in  action  at  the  same  time,  it  is| 
convenient  to  consider  them  separately. 

Mechanical  Disintegration. — Changes  of  temperature,  m 
between  day  and  night,  or  from  season  to  season,  cause  un 
equal  expansion  and  contraction  of  the  minerals  and  grain.<| 
of  which  rocks  are  composed.     Various  and  complex  stressc> 
are   thus   produced   which  cause  even   the   most  comp.u: 
granite  to   crumble.     The  freezing  of  water  contained  ir 
crevasses  in  rocks  or  in  the  interspaces  between  grains  ch 
crystals,    is   accompanied    by   expansion,    which    exerts 
powerful  force  tending  to  fracture  and  disintegrate  thi  n 


-« 


DTSINTEGRA  TION  AND  DECA  Y  OF  ROCKS 


.-I 


The  roots  of  trees  enter  crevices  in  the  rocks,  and  as  they 
enlarge,  force  off  fragments  frequently  of  large  size.  The 
undercutting  of  cliffs  and  banks  by  streams  and  by  the 
waves  and  currents  of  lakes  and  of  the  ocean,  causes  the  dis- 
lodgment  of  vast  quantities  of  earth  and  stone.  The  fall  of 
rocky  material  produced  by  these  and  still  other  causes, 
leads  to  still  further  breakage.  Rock  masses  are  also  loos- 
ened or  caused  to  fall  by  earthquake  shocks.  Volcanoes 
discharge  a  great  volume  of  fragmental  material  into  the 
air,  and  the  cooling  of  lavas  causes  them  to  become  frac- 
tured and  jointed.  When  molten  lava  enters  surface-water 
bodies,  steam  and  gas  explosions  occur,  and  the  rock  is 
perhaps  blown  to  dust.  Rain-drops,  snow  crystals,  and 
hail  by  beating  on  the  rocks  exert  a  force  tending  to  break 
off  fragments  loosened  by  other  and  principally  chemical 
agencies,  and  to  wear,  and  frequently  to  polish,  exposed 
surfaces.  Sand  and  dust,  blown  by  the  wind,  on  coming  in 
contact  with  rock  exposures,  wear  away  the  softer  parts  and 
loosen  the  harder  grains  and  crystals.  Glaciers  as  they  flow 
down  mountain  valleys  or  move  over  the  surface  of  more 
level  land,  tear  away  projecting  ledges,  and  when  charged 
I  with  sand  and  stones  abrade  and  grind  away  the  rock  over 
which  they  move.  Avalanches  and  landslides  rush  down 
declivities,  carrying  destruction  in  their  paths,  and  sweep 
[along  loosened  rock  fragments  which  are  broken  still  finer 
[and  in  part  ground  to  powder.  The  streams  themselves, 
mder  certain  conditions,  roll  along  stones  and  even  large 
)oulders,  which  become  rounded  and  broken,  at  the  same 
time  abrading  the  rocks  over  which  they  are  carried,  and 
has  aid  in  the  general  process  of  rock  disintegration  which 


1; 

II    1 


■■Ri 


RIVERS  OF  NORTH  AMERICA 


ii 


prepares  the  material  composing  the  land  for  stream  tran^i- 
portation. 

All  of  the  agencies  just  enumerated  are  mechanical  in 
their  action,  although  accompanied  by  chemical  changes, 
and  are  confined  to  the  surface,  or,  at  most,  to  an  extremely 
superficial  portion,  of  the  earth's  crust.  There  are  also  im- 
portant mechanical  agencies  which  act  deep  below  the  sur- 
face and  jead  to  the  fracturing  of  the  rocks  in  such  manner 
as  greatly  to  facilitate  the  agencies  producing  disintegration 
in  operation  at  the  surface.  And,  besides,  on  account  of 
the  continual  lowering  of  the  surface  in  many  regions  owing 
to  the  removal  of  material,  rock  fragments  originating  at  a 
greater  or  less  depth  become  mingled  with  those  produced 
at  the  surface  in  the  several  ways  just  enumerated,  and  thus 
become  of  interest  in  the  study  of  the  manner  in  which 
rocks  are  reduced  to  fragments  of  such  size  that  they  can 
be  moved  by  streams. 

Of  the  mechanical  agencies  leading  to  the  fracturing  of 
rocks  below  the  reach  of  frost  and  of  normal  changes  in 
temperature,  the  most  important  are  movements  in  the 
earth's  crust,  the  nature  of  which  it  is  impracticable  to  dis- 
cuss at  this  time,  which  cause  even  the  most  massive  layers 
to  become  folded  and  broken.  These  movements  are  fre- 
quently accompanied  by  the  crushing  of  the  rocks  in  zones 
of  various  widths,  as  when  a  fracture  is  formed  and  its  walls 
ground  against  each  other.  Such  breaks,  accompanied  or 
followed  by  differential  movements  of  their  walls,  are 
termed  faults,  and  the  rock  fragments  produced  are  desig- 
nated fault  breccias. 

The  world  over  and  to  a  great  but  indefinite  depth,  the 


DISINTEGRATION  AND  DECAY  OF  ROCKS 


5 


rocks  are  divided  by  what  are  known  as  joints,  the  origin  of 
which  is  obscure.  These  dividing  planes  are  similar,  we 
may  fancy,  to  gashes  made  by  a  sharp  blade  without  appre- 
ciable thickness,  drawn  through  the  rocks.  There  are  fre- 
quently two  series  of  joints  nearly  at  right  angles  tc5  each 
other,  and  more  or  less  nearly  vertical ;  these  are  intersected 
many  times  by  approximately  horizontal  cuts  of  the  same 
character,  and  frequently  also  by  planes  of  bedding.  The 
rocks  are  divided  in  this  manner  into  masses  that  are  some- 
times nearly  true  cubes.  In  many  instances  the  joints  cross 
each  other  irregularly  and  divide  the  rocks  into  blocks  of 
many  shapes.  Joint  blocks  of  whatever  form  vary  in  size, 
from  a  small  fraction  of  a  cubic  inch  to  many  cubic  feet.  In 
some  instances  these  are  contributed  directly  to  streams,  as 
when  they  fall  from  the  face  of  a  precipice,  but  more  com- 
monly they  are  broken  and  variously  modified  by  atmos- 
pheric agencies  before  being  fed  to  the  flowing  waters. 
The  joints  in  rocks,  although  of  inappreciable  width  deep 
below  the  surface,  are  planes  of  weakness  along  which 
chemical  and  mechanical  agencies  find  favourable  lines  of 
attack.  They  open  when  the  rocks  are  exposed  to  the 
weather,  and  greatly  favour  the  further  disintegration  re- 
sulting from  changes  of  temperature,  the  freezing  of  water, 
etc.  The  jointing  of  rocks  is  one  of  the  primary  and  most 
important  methods  by  which  they  become  divided  into 
blocks,  thus  exposing  greater  surfaces  to  the  attack  of 
chemical  agencies,  and  in  many  regions,  particularly  in 
ruf^gcd  mountains  and  in  canyon  walls,  exerts  a  direct  and 
pronounced  influence  on  topographic  forms. 
Among  the  agencies  that  lead  to  the  fracturing  and  me. 


RIVERS  OF  NORTH  AMERICA 


ill 


chanical  disintegration  of  rocks  deep  below  the  surface, 
should  be  noted,  also,  injections  of  molten  rocks  forced  up- 
ward into  the  earth's  crust,  earthquake  shocks,  the  friction 
of  debris  carried  by  subterranean  str-^ams,  and  the  falling  of 
cavern  roo^^s.  Still  another  agency,  as  has  been  pointed  out 
by  G.  P.  Merrill,  in  part  chemical  and  in  part  mechanical  in 
its  action,  results  from  the  combination  of  water  with  certain 
mineral  substances,  producing  what  is  termed  hydration. 
This  is  accompanied  by  an  increase  in  the  bulk  o^  the  min- 
erals affected,  and  t  o  consequent  production  of  stresses  in 
the  rocks  containing  them.  In  some  instances,  apparently 
unaltered  rock,  when  removed  from  mines  and  tunnels, 
rapidly  crumbles  from  this  cause  when  exposed  to  the  air. 
In  nature,  the  lowering  of  the  surface  by  erosion  and  the 
exposure  of  previously  deeply  buried  rocks  would  bring 
about  simikr  changes.  There  are  yet  other  alterations  in 
progress  in  the  rocks  due  to  chemical  action,  that  promote 
mechanical  disintegration,  which  cannot  be  noted  at  ^nis  time. 

By  the  several  processes  just  enumerated,  the  rocks  are 
broken  into  blocks  of  all  shapes  and  dimensions,  some  of 
which  are  of  the  size  of  gravel,  sand,  and  dust  grains,  and 
are  thus  rendered  suitable  for  stream  transportation,  and  to 
act  as  tools  by  means  of  which  flowing  water  promotes  the 
process  of  rock  breakage. 

Chemical  Disintegration  or  Rock-Decay. — WatvT  is  a  solv- 
ent for  probably  all  substances  that  occur  in  the  earth's 
crust,  although  in  many  instances  acting  with  extreme  slow- 
ness. The  readiness  with  which  mosw  substances  are  taken 
into  solution  by  water  is  enhanced  by  an  increase  of  tem- 
perature, and  in  nature  is  also  greatly  assisted  by  various 


DISINTEGRATION  AND  DECAY  OF  ROCKS 


substances,  especially  organic  acids,  with  which  it  becomes 
charged.  ; 

Even  rain-water  is  never  pure,  but  contains  various  salts  | 
and  gases  derived  from  the  a*".  Principal  among  these  is 
carbon  dioxide,  or  carbonic  acid.  Rain-water  on  reaching 
the  ea^'th  flows  over  the  surface,  or  percolates  for  a  time 
through  the  soil  and  rc"l^"  and  thus  comes  into  intimate  re- 
lations with  the  great  store  of  organic  acids  supplied  by  the 
wuste  and  decay  of  animal  and  vegetable  life.  The  chemi- 
cal energy  of  the  water  is  thus  greatly  enhanced,  and  it 
becomes  an  active  solvent  for  most  mineral  substances. 
Some  of  the  minerals  composing  rocks  are  more  soluble 
than  others,  and,  being  removed,  allow  those  that  remain 
to  crumble  and  fall  apart.  The  mineral  substances  taken  in 
solution  are,  for  the  most  part,  contributed  to  streams  and 
by  them  carried  to  thv^  sea  as  an  invisible  load ;  but  a  por- 
tion is  taken  below  the  surface  by  downward-percolating 
waters  and  undergoes  many  changes  in  composition  and  at 
the  same  time  produces  various  alterations  in  the  rocks 
through  which  it  passes. 

The  chemical  changes  produced  in  the  rocks  by  percolat- 
ing waters,  while  most  active  near  the  surface,  occur  also  at 
considerable  depths,  and  are  there  augmented  by  the  inter- 
nal heat  of  the  earth.  There  is  a  lower  limit  to  this  process, 
however,  due  to  the  increasing  density  of  the  rocks  with 
pressure  and  to  the  rise  of  temperature  with  increase  in 
depth.  There  are  good  reasons  for  concluding  that  surface 
waters  cannot  descend  more  than  twenty  thousand  or 
thirty  thousand  feet  below  the  surface. 

The  chemical  changes  due  to  percolating  water  are  influ- 


8 


RIVERS  OF  NORTH  AMERICA 


i"!i! 


enced  in  a  variety  of  ways  by  temperature.  The  rocks  are 
dissolved  most  readily  in  warm,  moist  regions.  It  is  in  such 
regions  also  that  vegetation  is  most  luxuriant  and  animal 
life  most  abundant,  and  hence  the  waters  are  most  highly 
charged  with  organic  acids.  Chemical  action,  in  most  in- 
stances, is  retarded  by  cold;  vegetatioii  is  less  abundant 
and  decay  less  rapid  in  cold  than  in  warm  climates;  it  is, 
therefore,  in  cold  regions  that  the  decay  of  the  rocks  is  at 
a  minimum. 

In  warm,  humid  countries,  deep  rock-decay  has  usually 
taken  place,  but  a  thick  surface  sheet  of  decomposed  ma- 
terial is  not  necessarily  found,  as  the  loosened  debris  may 
be  carried  away  as  fast  as  it  is  produced.     In  the  southern 
Appalachians,  and  in  many  other  warm  temperate  or  equa- 
torial regions,  the  rocks  are  so  broken  and  decayed,  even  at 
a  depth  of  one  hundred  and  fifty  or  two  hundred  feet  from 
/  the  surface,  that  they  may  be  crumbled  between  the  fingers 
;  or  moulded  like  clay.     In  such  instances  the  soil  usually 
shows  various  tints  of  red  and  yellow,  the  colours  being  due 
(  to  the  oxidation  and  hydration  of  iron  present  in  them. 
In  warm,  dry  countries  chemical  changes  in  the  surface 
material  are  retarded,  although  the  rocks  may  be  greatly 
shattered  by  changes  of  temperature.     In  such  regions  the 
soils  are  seldom  red. 

Chemical  changes  produced  by  percolating  water  below 
the  superficial  portion  of  the  earth — that  is,  below,  per- 
haps, one  hundred  feet — increase  with  depth,  on  account  of 
progressively  increasing  temperature,  but  these  changes 
are  beyond  the  immediate  subject  under  discussion.  An 
important  agency  in  rock  disintegration  and  decay  having 


Its  so 
cially 


DISINTEGRATION  AND  DECAY  OF  ROCKS 


its  source  deep  within  the  earth,  however,  is  mr iiifest  espe- 
cially in  volcanic  regions  where  steam  charged  with  various 
acids  rises  through  fissures  and  other  openings.  During 
volcanic  eruptions,  but  more  particularly  after  a  volcano  has 
passed  to  the  condition  of  a  fumarole  or  a  solfatara,  heated 
vapour  and  gases  charged  with  sulphuric,  hydrochloric,  car- 
bonic, and  other  acids  escape  in  large  volumes,  sometimes 
continuously  for  centuries,  and  produce  conspicuous  changes 
in  the  rocks  through  which  they  rise.  Similar  but  usually 
less  copious  exhalations  occur  from  lava  streams,  and  pro- 
duce alterations  in  the  lava  which  influence  the  character  of 
the  soil  resulting  from  them. 

The  chemical  alterations  produced  by  percolating  water, 
and  less  commonly  by  volcanic  gases,  in  rocks  near  the 
surface,  are  in  part  by  solution,  and  in  part  oxidation, 
hydration,  precipitation,  etc.  These  changes,  except  the 
last  mentioned,  may  be  grouped,  at  least  in  a  general  way, 
under  the  term  rock-decay.  In  decaying,  the  rocks  are 
more  or  less  disintegrated,  however,  since  the  mere  soluble 
minerals  are  removed,  thus  allowing  the  less  soluble  rock 
constituents  to  crumble  and  fall  apart.  The  processes  of 
rock-disintegration  and  rock-decay  mutually  assist  each 
other,  and  progress  at  the  same  time.  The  result  is  that 
the  surface  layer  of  the  earth's  crust  is  profoundly  altered, 
and  a  sheet  of  modified  material  is  produced,  which  is 
designated  in  part  as  soil  and  in  part  as  rock  detritus." 

The  Removal  and  Rencival  of  Surface  Debris. — The  sur- 


'  The  name  Regolith,  meaning  blanket-stone,  has  recently  bee.,  proposed  for 
the  superficial  material  coverin'T  the  earth,  by  G.  I*.  Merrill,  A  Treatise  on 
Rocks,  Rock- Weathering  and  Soils ,  1897,  p.  2y9. 


lO 


RIVERS  OF  NORTH  AMERICA 


ill! 


ill! 


face  changes  just  considered  have  been  in  progress  since  the 
first  appearance  of  land,  and  will  continue  as  long  as  conti- 
nents and  islands  exist.  In  former  geological  periods  the 
agencies  enumerated,  particularly  those  of  a  chemical  nature, 
were  more  active  than  now,  and  have  varied  from  time  to 
time,  in  probably  all  portions  of  the  earth's  surface,  with 
climatic  and  other  changes.  Throughout  all  geological  ages, 
the  streams  have  been  actively  engaged  in  removing  the  dis- 
integrated and  more  or  less  chemically  altered  surface  por- 
tions of  the  earth's  crust.  In  places  and  at  certain  times, 
the  debris  has  been  removed  as  fast  as  formed,  and  bare, 
hard  rock  surfaces  have  been  exposed ;  at  other  times,  the 
supply  has  been  in  excess  of  the  demand,  and  deep  accumu- 
lation^ have  resulted.  The  surface  sheet  of  debris  has  been 
continually  wastiog  and  continually  renewed.  Throughout 
the  history  of  the  earth,  topographic  changes  have  been  in 
progress.  Mountain  ranges  and  systems  have  been  upraised, 
the  rocks  composing  them  fractured  and  chemically  altered, 
and  borne  away  by  streams.  Where  once  a  magnificent 
mountain  range  reared  its  battlements  among  the  clouds, 
there  is  now  a  plain  but  little  elevated  above  the  sea.  Not 
only  one,  but  several  such  geographical  cycles  have  run  their 
courses  in  many  lands. 

During  the  cycles  still  in  progress  human  agencies  have 
been  added  to  those  previously  in  action.  This  new  element 
in  the  earth's  history  has  become  more  and  more  important 
as  man  has  advanced  in  civilisation.  In  part,  human  indus- 
tries have  retarded  the  work  of  physical  and  chemical  agen- 
cies, but  in  the  main  man  has  been  a  destroyer. 

The  removal  of  the  portion  of  the  earth's  crust  rising 


DISINTEGRA  TION  AND  DECA  Y  OF  ROCKS 


II 


above  the  sea,  during  each  cycle,  has  been  done  almost 
wholly  by  streams.  The  manner  in  which  the  rocks  are 
prepared  for  transportation,  however,  is  quite  as  important 
to  the  geographer  as  the  methods  employed  for  their  re- 
moval, but  the  brief  review  given  above  of  this  division  of 
the  general  process  must  suffice  for  the  present. 

The  student  who  may  wish  to  continue  the  studies  out- 
lined in  this  chapter  will  find  assistance  in  the  books  men- 
tioned below.' 

'  George  P.  Merrill.  A  Treatise  on  Rocks,  Rock-  Weathering  and  Soils, 
pp.  172-398.     The  Macmillan  Co.,  1897. 

Israel  C.  Russell.  The  Decay  of  Rocks  and  the  Origin  of  the  Red  Colour 
of  Certain  Formations.     U.  S.  GeologicJ  Survey,  Bulletin  No,  52,  1889. 

Israel  C.  Russell.  "A  Reconnoissance  in  South-Eastern  Washington." 
U.  S.  Geological  Survey,  Water-Supply  and  Irrigation  Papers,  No.  4,  pp.  57- 
69,  1897. 

George  P.  Marsh.  The  Earth  as  Modified  by  Human  Action.  Charles 
Scribner's  Sons,  1885. 

John  C.  Branner.  "  Decomposition  of  Rocks  in  Brazil."  Bulletin  of  the 
Geological  Society  of  America,  vol.  vii.,  pp.  255-314,  1896. 

Alexis  A.  Julien.  "On  the  Geological  Action  of  the  Humus  Acids,"  in 
American  Association  for  the  Advancement  of  Science,  Proceedings,  vol.  xxviii., 
pp.  311-410,  1879. 

Walter  Maxwell.  Lavas  and  Soils  of  the  Hawaiian  Islands.  Honolulu, 
1898. 


;■;  i   :■ 


'      ■-<■<    ■  •■ ' 


CHAPTER  II 


LAWS  GOVERNING  THE  STREAMS 


wm 


■■'•Mxm 


m 


'III 
I  ''>i<i'i 


THE  water  which  flows  off  from  the  land,  as  is  well 
known,  is  supplied  by  the  condensation  of  vapour  in 
the  air.  A  part  of  the  water  reaching  the  earth  flows  over 
the  surface  and  gathers  into  rills  which  unite  to  form  larger 
streams,  and  a  part  shiks  below  the  surface  and,  after  follow- 
ing an  underground  course,  usually  by  percolation  through 
porous  soil  or  rocks,  emerges  in  springs,  many  of  which  join 
the  surface  flow. 

It  is  also  well  known  that  streams  ranging  in  size  from  the 
smallest  rills  to  the  mightiest  rivers  are  engaged  either  oc- 
casionally, as  during  floods,  or  continually,  in  carrying  away 
material  that  was  previously  a  portion  of  the  land.  The 
manner  in  which  this  material  is  acquired  by  the  streams, 
the  way  it  is  transported,  the  effects  it  has  on  the  flow  of 
the  streams,  and  on  their  bottoms  and  sides,  the  modifica- 
tions in  the  configuration  of  the  surface  of  the  land  due  to 
the  removal  and  re-deposition  of  debris,  etc.,  are  all  phe- 
nomena that  obey  definite  laws  and  are  variously  modified 
■  by  conditions.  If  one  can  ascertain  the  laws  governing  the 
^behaviour  of  a  single  stream,  they  should  also  apply  no': 
only  to  the  streams  of  North  America,  but  to  those  of  all 
land  areas. 

IS 


nllii 


l:''i 


Fig.  B.     Ingall's  Creek,  Washington. 
Showing  boulders  too  large  for  the  stream  to  move  even  during  high-water  stages. 


i 


No« 

dividuj 

tration 

life  of 

ever,  a 

sketch 

phases 

its  worl 

How 

earth  w 

tance  t 

etc,     II 

fee  ther 

beating 

able  fon 

the  dro| 

clear  Wc 

ploughe 

disturbe 

in  obedii 

held  in 

these  agi 

disturbec 

rills  to  tl 

halts,  to 

come  mi 

the  moui 

during  e^ 

cient  timi 

nitude   b( 


I  i, 


LAIVS  GOVERNING  THE   STREAMS 


13 


No  one  stream,  perhaps,  in  the  limited  time  that  an  in- 
dividual student  is  enabled  to  examine  it,  will  furnish  illus- 
trations of  all  of  the  modifying  conditions  influencing  the 
life  of  a  great  river.  By  selecting  typical  examples,  how- 
ever, affected  by  different  modifying  conditions,  we  may 
sketch  a  composite  picture  which  will  represent  the  various 
phases  in  the  life  history  of  a  single  river  that  has  carried  on 
its  work  for  tens  of  thousands  of  years. 

How  Streams  Obtain  their  Loads. — Rain-drops  strike  the 
earth  with  a  certain  force,  dependent  on  their  size,  the  dis- 
tance they  descend,  the  direction  and  force  of  the  wind, 
etc.  If  rain-drops  fall  on  the  surface  of  a  still  pool  we  may 
.cee  them  rebound.  If  we  face  a  rain-storm,  the  sting  of  the 
beating  drops  again  assures  us  that  they  exert  a  consider- 
able force  on  the  objects  against  which  they  strike.  When 
the  drops  fall  on  a  solid  rock  surface,  they  gather  in  rills  of 
clear  water,  but  if  they  fall  on  loose  soil,  as  a  newly 
ploughed  field,  for  example,  the  finer  particles  of  earth  are 
disturbed,  and  as  the  waters  gather  into  rills  and  flow  away 
in  obedience  to  gravity,  they  are  turbid  with  earth  particles 
held  in  suspension.  The  turbid  rills  unite  in  brooks,  and 
these  again  combine  to  form  larger  streams.  The  fine  silt 
disturbed  by  the  impact  of  the  rain-drops  is  carried  by  the 
rilL  to  the  brooks  and  thence  onward,  perhaps  with  many 
halts,  to  the  sea.  After  heavy  rains  even  large  rivers  be- 
come muddy.  The  lakes  and  large  areas  in  the  sea  near 
the  mouths  of  rivers  are  then  discoloured.  This  happens 
during  every  storm  the  world  over,  and  evidently,  if  suffi- 
cient time  be  allowed,  must  lead  to  changes  of  great  mag- 
nitude  both   in  the  topography  of  the  land  from  which 


•kT^ 


t 


14 


RIVERS  OF  NORTH  AMERICA 


material  is  removed  and  in  the  shape  of  the  basins  where  it 
is  deposited.  :  • 

When  the  surface  of  the  land  is  dry,  and  especially  when 
bare  of  vegetation,  earth  particles  are  moved  by  the  wind  in 
much  the  same  manner  that  streams  take  up  and  transport 
the  flakes  and  grains  of  rock  which  they  are  competent  to 
transport.  The  dust  and  sand  carried  by  the  wind  and 
falling  in  streams  is  another  source  from  which  they  obtain 
material  suitable  for  removal. 

The  fine  rock  powder,  or  glacial  meal  as  it  is  termed, 
produced  by  the  grinding  of  stones  held  in  the  ice  against 
each  other  and  on  the  rocks  over  which  the  glaciers  flow,  is 
contributed  directly  to  the  waters  formed  by  the  melting  of 
the  ice.  For  this  reason  nearly  every  glacier-born  stream  is 
turbid  and  heavy  with  silt. 

Volcanoes  during  times  of  violent  eruption  discharge  vast 
quantities  of  fine  dust,  and  in  many  instances  equally  abund- 
ant rock  fragments  of  the  size  of  sand  and  gravel.  When 
material  extruded  in  these  forms  falls  in  streams,  or  is  car 
ried  in  by  tributary  rills  and  by  the  wind,  another  source  is  fur 
nished  from  which  streams  receive  their  initial  loads.  There 
are  yet  other  methods  by  which  streams  are  supplied  with 
material  in  a  suitable  condition  to  be  transported.  Among 
these  may  be  noted  the  fall  of  cosmic  dust,  disturbances 
produced  by  avalanches  and  landslides,  the  uprooting  of 
tr^es,  the  impact  of  driftwood  and  floating  ice  on  the  bot- 
toms and  sides  of  stream  channels,  the  movement  of  roots 
and  overhanging  branches  by  the  wind  and  by  the  currents 
of  the  streams  themselves,  disturbances  of  the  material 
forming  the  bottoms  and  sides  of  stream  channels  by  ani- 


LAWS  GOVERNING  THE   STREAMS 


15 


mals, — as  by  beavers,  for  example, — contributions  of  shells 
and  siliceous  cases  from  organisms  like  mollusks  and  diatoms 
living  in  the  waters,  etc.  Man  promotes  the  transfer  of 
solid  matter  to  the  streams  in  variou'--  ""ays,  more  especially 
by  ploughing  and  otherwise  disturbing  the  soil,  and  by  the 
removal  of  forests.  All  of  the  methods  mentioned  in  this 
paragraph,  however,  are  of  secondary  importance  in  com- 
parison with  the  influence  of  rain,  wind,  and  glaciers. 

The  particles  carried  in  suspension  by  streams  tend  to 
fall  to  the  bottom,  being  continually  pulled  down  by  grav- 
ity, but  in  flowing  water  there  are  various  currents,  some  of 
which  tend  upward  and  exert  an  influence  on  the  falling 
particles  in  opposition  to  gravity.  The  currents  in  the 
water  move  the  suspended  particles  in  various  directions 
and  retard  their  fall  to  the  bottom,  but  the  resultant  move- 
ment is  in  the  direction  of  the  flow  of  the  stream. 

The  particles  carried  by  streams  fall  to  the  bottom  many 
times  during  their  journeys,  and  rest  there  for  a  period  per- 
haps brief  but  possibly  long,  and  are  again  lifted  by  upward 
currents  and  brought  within  the  influence  of  the  onward 
flow.  Material  thus  transported  by  a  stream  may  for  con- 
venience be  termed  its  load.  Streams  not  only  receive  their 
initial  loads  in  the  various  ways  just  stated,  but  there  are 
other  methods  by  which  the  same  result  is  reached.  As 
will  be  considered  later,  flowing  water  exerts  a  pressure  on 
objects  against  which  it  strikes.  If  this  force  be  great 
enough  to  move  the  objects  in  the  path  of  a  stream,  they 
will  be  pushed  along,  rolled  over,  or,  with  the  assistance  of 
upward  currents,  taken  in  suspension.  The  strength  of  the 
water  cunent    determines  the  size  of  the  particles  it  can 


\% 


1^1 


J 


V 


\ 


I6 


RIVERS  OF  A  OR  77/  AMERICA 


carry,  so  that  a  stream  of  a  given  velocity,  but  without  an 
initial  load,  would  be  able  to  remove  from  its  channel  all  of 
the  loose  particles  it  is  competent  to  carry  and  thereafter 
would  run  clear  unless  its  velocity  were  increased. 

The  principal  methods  by  which  streams  receive  their 
initial  loads  insure  a  waste  of  the  land  between  the  drain- 
age lines,  and  consequently  this  land  changes  in  topographic 
form.  The  deepening  and  broadening  of  the  stream  chan- 
nels is  accomplished  principally  by  the  friction  of  the  debris 
carried  through  them,  aided  also  by  solution.  The  laws 
governing  this  complex  process  will  be  considered  I'^r-r. 

Transportation. — The  debris  acquired  by  str  .in  ,.  ii.  the 
several  ways  considered  above  is  carried  along  by  them.  A 
convenient  term  for  this  process  is  transportation.  Light 
objects  like  leaves,  wood,  pumice,  etc.,  are  floated  on  the 
surface,  but  meet  with  various  delays,  and  undergo  more  or 
less  chemical  and  mechanical  changes  during  their  journeys, 
and  sooner  or  later  sink  to  the  bottom.  Material  like  fine 
sand  and  silt  remains  in  suspension  to  a  great  extent  and  is 
carried  bodily  onward.  Heavier  objects,  like  pebbles  and 
boulders,  are  either  rolled  or  pushed  along  the  bottom,  o- 
remain  at  rest  until  they  are  reduced  in  size  by  the  frictit  ' 
and  solution,  or  shattered  by  the  impact,  of  material  swept 
against  them.  The  size,  weight  (specific  gravity),  and  form 
of  the  loose  material  within  the  influence  of  a  stream  deter- 
mine whether  or  not  it  will  be  moved  by  a  current  of  a 
given  velocity.  The  smaller  the  divisions  into  which  a  mass 
of  rock  is  broken,  the  larger  the  ratio  of  surface  to  weight. 
The  force  which  a  current  of  a  given  velocity  exerts  against 
objects  in  its  path  varies  as  the  area  of  the  opposing  .u«r- 


,/     ; 


LAPVS  GOVERiYING  THE   STREAMS 


m 


face.  The  smaller  the  parts  into  which  a  rock  mass  be- 
comes divided,  therefore,  the  greater  the  tendency  of  the 
current  to  move  them. 

The  ability  of  the  stream  to  carry  debris  in  suspension, 
however,  depends  not  only  on  its  velocity  and  the  degree  of 
comminution  of  the  material  within  its  influer^'i,  but  also, 
as  previously  stated,  on  the  presence  of  secondary  and  es- 
pecially of  upward  currents  which  tend  to  lift  the  particles 
brought  within  their  influence.  While  a  particle  is  in  sus- 
pension the  onward  currents  bear  it  along,  bu':  gravity  is  all 
tho  while  acting,  and,  unless  counteracted,  finally  pulls  it 
to  the  bottom.  The  journey  of  a  rock  fragment  from  the 
mountains  to  the  sea  consists  of  a  great  number  of  upward 
and  onward  excursions,  with  rests  of  greater  or  less  length 
between.  More  dcfinilely,  the  ability  of  a  stream  to  hold 
debris  in  suspension  is  due  to  the  fact  that  different  layers 
of  water  are  actuated  by  different  velocities,  and  these  exert 
diHerent  pressures  upon  the  different  sides  of  the  suspended 
particles.  Hence,  the  greater  the  differences  in  the  veloci- 
ties of  consecutive  layers,  the  greater  will  be  the  tendency 
to  hold  material  in  suspension.  It  is  stated  by  Humphreys 
and  Abbot,  from  whose  report  on  the  Mississippi  much  of 
this  discuf  jion  of  the  mechanics  of  stream  flow  is  taken, 
that  the  change  in  the  velocity  of  the  waters  of  streams  in 
horizontal  planes  is  greatest  near  the  shore  and  bast  near 
the  thread  of  maximum  current;  and  in  vertical  planes,  is 
greatest  near  the  bottom  and  surface  and  least  at  about  one- 
third  of  the  depth  of  the  stream — that  is,  where  the  abso- 
lute velocity  is  greatest.  If.  then,  the  water  be  cither 
charged  to  its  maximum  caj'  icity  or  overcharged  with  sedi- 


^*k 


■*ft 


I 

I 

lit 


t 


I8 


RIVERS  OF  NORTH  AMERICA 


merit,  the  highest  percentage  of  material  in  suspension  will 
be  found  near  the  banks  and  near  the  surface  and  bottom, 
and  the  least  amount  near  the  thread  of  the  maximum  cur- 
rent and  at  a  depth  of  about  one-third  of  that  of  the  stream. 
If,  however,  the  water  is  undercharged  with  material  in 
suspension,  the  distribution  will  not  follow  any  law,  the 
amount  at  any  locality  being  determined  by  what  may  be 
considered  as  accidental  swirls,  boils,  etc.  As  most  streams 
are  undercharged,  it  follows  that  samples  of  water  from 
several  points  in  a  cross-section  should  be  examined  in 
order  to  ascertain  approximately  the  amount  of  material 
that  is  being  carried.  Rock  fragments  too  heavy  to  be 
lifted  may  be  rolled  or  pushed  along  the  bottom,  or  perhaps 
turned  over  from  time  to  time  by  the  resultant  onward  cur- 
rent. There  is  thus  an  adjustment  between  the  strength  of 
the  current  and  the  specific  gravity  of  the  material  trans- 
ported. 

It  is  well  known  that  the  power  flowing  water  has  to  trans- 
port rock  debris  increases  with  increase  of  velocity.  Experi- 
ments have  shown  that  if  water  is  mndcj  to  flow  through  an 
even  channel  and  the  rate  of  flow  is  gradually  increar.ed  by 
increasing  the  inclination  of  the  channel,  it  will  move  ma- 
terial added  to  it  approximately  as  follows  ' : 


VELOCITY   OF  CURRENT. 

3  inches  per  second. 

6  "       "        '• 

12  "       "        " 

2  feet      " 


SIZE   OK   MATERIAL   MOVED. 

Fine  clay  and  silt. 

Fine  sand. 

Pebbles  \  inch  in  diamrter. 

I 


'David  Stevenson,  Canal  and  River  Engineering,  p.  315;  A.  J.  Jukes- 
Browne,  Physical  Geology,  i8q2,  p.  130 ;  Archibald  (Jeikie,  Text-Book  of 
Geology,  2d  edition,  1885,  p.  354;  Joseph  Le  Conte,  Elements  of  Geology. 
4th  edition,  1896,  pp.  i5-20.     See  also  other  elementary  works  on  geology. 


ZAtVS  GOVERNING  THF   STREAMS 


19 


'ELOC 

TY   OF   CURRENT. 

SIZE   OF 

MATERIAL   MOVED. 

2.82 

feet  per 

second. 

Pebbles  2 

inches  in  diameter 

3.46 

i  I 

3 

4 

it 

4 

4.47 

•* 

5 

4.90 

41 

6 

5-29 

It 

7 

5.65 

1 1 

8 

It  must  be  understood  that  the  currents  referred  to  in 
this  table  are  bottom  currents,  and  in  general  may  be  taken 
at  about  one-half  the  central  surface  current. 

An  important  fact  shown  by  these  and  other  similar  ex- 
periments is  that  the  transporting  power  of  running  water 
increases  in  a  greater  ratio  than  the  increase  in  velocity. 

It  has  been  demonstrated  that  if  the  surface  of  an  object 
opposed  to  a  current  of  water,  as  the  pier  of  a  bridge,  for 
example,  remains  constant,  the  force  of  current  striking  it 
varies  as  the  square  of  its  velocity.  Also,  that  the  trans- 
porting power  of  a  current,  or  the  weight  of  the  largest  frag- 
ment it  can  carry,  varies  as  the  sixth  power  of  the  velocity.^ 
Under  this  law  it  will  be  seen  that  doubling  the  velocity  of 
a  current  increases  its  transporting  power  si>:ty-four  times. 
If  a  stream  flowing  with  a  given  velocity  is  able  to  move 
stones  weighing  one  pound,  by  doubling  the  velocity 
boulders  weighing  sixty-four  pounds  can  be  carried ;  and  if 
the  velocity  were  increased  ten  times,  rocks  weighing  one  mil- 
lion pounds  could  be  moved.  This  enables  us  to  see  how 
streams  are  capable  of  producing  such  striking  results  during 
floods,  when  their  velocities  are  increased  on  account  of  an 
increase  in  volume.     The  gradient  of  a  river,  or  its  average 

'  A  demonstration  of  this  proposition  may  be  found  in  Joseph  Lc  Conte's 
Eliniftifs  tf  Giology,  4th  edition,  pp.  19,  20,     Appleton  &  Co.,  1896. 


20 


RIVERS  OF  NORTH  AMERICA 


fall  in  a  given  distance,  as  a  rule,  progressi  '^ely  decreases 
fro.n  near  its  source  to  its  mouth.  With  this  general  de- 
crease in  gradient  there  is  a  decrease  in  velocity,  and  con- 
sequently a  loss  in  transporting  power  and  a  diminution  or 
total  check  of  frictic  .  on  the  stream's  bottom.  As  a  result 
of  these  conditions,  we  usually  find  that  the  streams  are 
actively  engaged  in  deepening  their  channels  in  their  upper 
courses,  and  are  consequently  able  to  extend  their  branches 
farther  and  farther,  thus  acquiring  new  territory,  and  at  the 
same  time  to  deposit  material  in  their  lower  and  less  steep 
courses  nearer  their  mouths.  Variations  in  this  process  oc- 
cur not  only  from  season  to  season  but  from  day  to  day,  on 
account  principally  of  variation  in  velocity  due  to  changes 
in  volume. 

The  carrying  of  debris  consumes  some  of  the  energy  of 
flowing  water.  As  an  extreme  example,  it  is  readily  seen 
that  an  excessive  quantity  of  fine  mud  contributed  to  r 
stream  will  entirely  check  its  flow.  If  but  little  mud  is 
added,  however,  it  is  carried  forward  without  sensibl}- 
diminishing  the  strength  of  the  current.  Without  attempt- 
ing to  present  a  complete  analysis  of  the  laws  governin^r 
stream  transportation,  it  will  be  sufficient  at  this  time  to 
note  that  streams  exert  a  selective  power,  taking  up  and 
carrying  forward  the  finer  and  lighter  material  within  their 
reach,  and,  if  this  be  sufficient  to  consume  their  available 
energy,  leaving  the  larger  and  heavier  masses,  although 
they  may  not  be  too  heavy  to  be  removed  if  the  energy  of 
the  stream  is  not  otherwise  taxed. 

The  principal  laws  governing  stream  transportation  may 
be  briefly  formulated  as  follows: 


LAIVS  GOVERNING  THE   STREAMS 


21 


1.  The  greater  the  slope  of  a  stream  channel,  the  greater 
the  amount  of  material  in  suspension  the  stream  can  carry ; 
the  reason  being  that  the  greater  the  slope  the  swifter  is  the 
flow  of  the  water  descending  it,  other  conditions  remaining 
unchanged.  The  increase  in  transporting  power  with  in- 
crease  of  slope  is  greater  than  a  single  ratio.  That  is,  if  the 
declivity  of  a  stream  is  double,  its  transporting  power  is 
more  than  double.        .        '       >^    ■      : 

2.  An  increase  in  the  volume  of  a  stream  increases  its 
ability  to  transport.  The  greater  the  volume  of  a  stream, 
the  greater  will  be  its  velocity,  and  the  less  its  loss  of  power 
due  to  friction  in  proportion  to  its  energy j,-'  Here,  again,  the 
increase  in  transporting  power  is  greater  than  a  simple  ratio. 

3.  The  capacity  of  a  stream  to  transport  is  greater  for 
fine  debris  than  for  coarse ;  for  the  reason  that  to  move  fine 
material  requires  less  power  for  the  same  weight  than  for 
coarser  material,  and,  also,  when  the  material  is  fint  a 
greater  portion  of  the  stream's  energy  can  be  utilised  than 
when  the  load  is  coarse.,, . 

One  of  the  most  important  principles  connected  with 
stream  transportation  is  that  flowing  water  assorts  the 
debris  delivered  to  it.  Fine  particles  are  more  easily  carried 
than  coarser  ones  of  the  '^ame  specific  gravity,  and  are  first 
removed.  This  is  true  both  of  particles  in  suspension 
and  of  material  rolled  along  the  bottom.  If  the  fine  ma- 
terial is  sufficient  to  consume  the  available  energy  of  a 
stream,  all  coarser  debris  is  left  until  its  energy  is  increased, 
as  during  storms,  or  until  the  fragments  too  large  to  be  re- 
moved are  reduced  in  size.  There  is  a  delicate  adjustment 
between  the  velocity  of  a  stream  and  the  size  of  the  debris 


22 


RIVERS  OF  NORTH  AMERICA 


m 

iiil 


it  can  carry,  which  may  be  termed  the  selective  power  of 
currents.  The  influence  of  this  selective  power  is  seen  not 
only  in  the  character  of  the  material  moved  by  streams,  and 
in  the  debris  left  on  their  bottoms,  but  in  the  deposits  which 
they  make,  whether  on  their  border,  or  in  the  lakes  and  sea 
to  which  they  contribute  their  loads.  As  most  sedimentary 
rocks  are  formed  of  stream-born  debris,  this  assorting  pro- 
cess must  evidently  be  of  vast  geological  importance. 

Debris  Carried  by  Ice. — An  interesting  and  at  times  an 
important  factor  in  stream  transportation  is  the  assistance 
furnished  by  ice,  which  frequently  enables  streams  to  move 
objects  that  would  otherwise  exceed  their  power. 

During  winter  the  water  of  streams  frequently  freezes  to 
the  bottom,  more  especially  along  their  margins,  and  stones, 
gravel,  sand,  etc.,  forming  the  beds  of  their  channels,  be- 
come firmly  ttached  to  the  ice.  In  spring,  when  the 
streams  are  swollen,  the  ice,  on  account  of  its  buoyancy, 
breaks  away  from  the  bottom,  but  frequently  retains  large 
quantities  of  debris,  which  is  carried  with  it  down-stream, 
and  may  make  a  long  journey  before  being  dropped  or  de- 
posited by  the  stranding  of  the  ice.'  Stones  carried  in  this 
manner  are  frequently  of  large  size.  A  rudely  spherical 
boulder  measured  by  me  on  the  bank  of  the  Yukon,  which 
had  certainly  travelled  scores  of  miles  from  its  parent  ledge, 
was  a  little  over  six  feet  in  diameter.  Many  others  very 
nearly  as  large  were  seen  which  had  recently  been  forced 
several  yards  up  the  banks  of  the  river.     All  of  these  were 

'  An  account  of  the  method  of  transportation  here  discussed  may  be  found  in 
Lyell's  Principles  of  Geology  (nth  edition,  vol.  i.,  pp.  359-3O3,  Appleton  &  Co., 
1873),  accompanied  by  an  illustration  of  large  boulders  alon^r  the  shores  of  the 
St.  Lawrence,  which  had  been  moved  through  the  agency  of  ice. 


far  be 
tion,  I 


,LAWS  GOVERNING  THE   STREAMS 


23 


far  beyond  the  reach  of  former  glaciers,  and,  without  ques- 
tion, had  been  deposited  in  their  present  positions  at  a  very 
recent  date ;  some  of  them,  in  fact,  during  the  floods  of  the 
preceding  spring. 

Another  method  by  which  the  ice  of  a  large  river  some- 
times becomes  freighted  with  debris  may  be  observed  where 
high-grade  tributaries  occur.  In  such  an  instance,  when 
spring  approaches,  the  small  streams  may  first  become  freed 
of  ice  and  be  able  to  sweep  down  debris  upon  the  still  frozen 
surface  of  the  river.  Again,  when  a  river  is  bordered  by 
steep  bluffs,  material  loosened  from  the  faces  of  the  cliffs 
falls  upon  the  ice  and  is  ready  for  removal  when  freshets 
occur.  Avalanches  may  also  bring  debris  to  a  frozen  river 
in  the  same  way  that  they  do  to  glaciers. 

The  assistance  in  transportation  rendered  to  streams  by  ice 
is,  of  course,  greatest  in  high  latitudes,  but  is  not  inconsider- 
able as  far  south  as  Virginia.  Along  the  Potomac  there  are 
frequently  boulders  much  too  large  for  the  unaided  waters 
to  move,  and  which  it  is  presumed  have  been  buoyed  up  by 
ice  during  a  part  at  least  of  their  journeys,  since  they  are 
well  beyond  where  the  river  loses  velocity  on  passing  from 
its  high-grade  upper  course  to  the  plain  near  the  sea.  In 
the  method  of  transportation  here  considered  it  is  not  neces- 
sary that  a  river  should  freeze  from  side  to  side.  The  ice 
that  forms  about  a  partially  submerged  boulder  near  the 
shore  of  a  stream  tends  to  buoy  it  up.  When  the  water 
surface  is  raised,  if  sufficient  ice  has  formed  about  the  stone, 
it  will  be  floated  away.'     In  the  terraces  of  the  Potomac 

'  The  weight  of  a  cubic  foot  of  water  at  32°  F.  is  62.417  pounds;  a  cubic 
foot  of  ice  weighs  5/. 2  pounds. 


t  ■ 
I 


If 

tiC 


t 


24 


RIVERS  OF  NORTH  AMERICA 


about  Washington,  there  are  boulders  two  to  three  feet  or 
more  in  diameter,  resting  on  fine  sand  and  clay,  which  it  is 
thought  wc/e  attached  to  ice-cakes  at  the  time  of  their  re- 
moval to  their  present  sites.  The  locality  referred  to,  it 
will  be  remembered,  is  south  of  the  southern  limit  of  former 
glaciers.  . 

The  stone-charged  ice  carried  each  spring  by  the  rivers  in 
high  latitudes  acts  much  like  a  glacier  in  grinding  the  bot- 
tom and  sides  of  the  channel  down  which  it  moves.  In  the 
case  of  a  large  river  this  action  is  most  pronounced  near  its 
borders,  where  the  water  is  shallow,  and  probably  does  not 
occur  at  all  in  the  deeper  portions.  On  the  border  of  Por- 
cupine River,  Alaska,  I  have  seen  large  areas  exposed 
during  low  water  where  the  bottom  consisted  of  stones  em- 
bedded in  tenacious  clay  so  as  to  form  a  veritable  pavement. 
The  ice  charged  with  debris  had  previously  moved  over  this 
pavement,  and  not  only  pressed  down  the  stones  so  as  to 
produce  a  generally  even  surface,  but  ground  their  exposed 
portions  so  as  to  make  facets  which  were  polished  and 
striated.  The  pebbles  and  in  some  cases  flat  stones  a  foot 
or  more  in  diameter,  bearing  these  markings,  have  a  remark- 
able resemblance  to  glaciated  boulders. 

In  other  instances  along  the  Yukon  I  found  the  solid 
rock,  on  prominent  points,  to  a  height  of  twenty  feet  or 
more  abo  the  summer  level  of  the  river,  smoothed  and 
striated  by  the  action  of  river  ice  in  much  the  same  manner 
that  is  familiar  in  formerly  glaciated  valleys.' 

During  spring  floods  in  northern  rivers,  ice-blocks  are  fre- 


'I.  C.  Russell,  "  Notes  on  the  Surface  Geology  of  Alaska,"  in  Bulletin  of 
the  Geological  Society  of  America,  vol.  i.,  pp.  n6-i22,  1890. 


Plate  ;I. 


X 


I**' 


■  Vi"ws  on  the  Yukon,  Alaska. 

A.— Looking  frori  the  river  across  a  portion  of  its  delta. 
B. — River-bank  Oi'  perennially  frozen  Eravel. 
C.  and  D. — Stones  left  by  Hoating  ice  during  spring  floocis. 
E. — Bluff  of  hard  rock  on  tho  border  of  a  deeply  cut  valley. 
F. — Small  cut-terraces  in  sand  deposited  during  high-water. 


quer 

flatc 

have 

in  th 

curio 

these 

basin 

Th 

in  mo 

ates  i 

the  fc 

termei 

especi 

and  CO 

botton 

to  lift 

in  carr; 

An  i 

one  of 

i869-7( 

the  en£ 

count,  ( 

-atones  ] 

but,  \vh 

thicknes 

stream  ^ 

count  is 

"The 
runs  over 


Ml! 


LAIVS  GOVERNING  THE  STREAMS 


25 


quently  stranded  and  even  forced  far  up  the  banks.  Where 
flat  cakes  of  ice  accumulate  in  this  manner,  they  sometimes 
have  gravel  and  sand  washed  over  them ;  this  material  lodges 
in  the  cracks  and  openings  between  the  cakes,  and  is  left  in 
curious  heaps  and  ridges  when  the  ice  is  melted.  Sometimes 
these  deposits  surround  small  areas  so  as  to  make  shallow 
basins.  .;-■;■:■■'■  ■■•,.•  ,v 

There  is  yet  another  method  by  which  ice  assists  streams 
in  moving  debris  down  their  channels,  and  one  which  oper- 
ates in  midstream,  where  the  current  is  swift.  I  refer  to 
the  formation,  during  excessively  cold  weather,  of  what  is 
termed  anchor  ice,  or  ground  ice,  at  the  bottom  of  streams, 
especially  where  the  waters  pli  age  over  small  obstructions 
and  comparatively  quiet  bottom-eddies  are  produced.  This 
bottom  ice  forms  about  stones,  and  by  its  buoyancy  tends 
to  lift  them  from  the  bottom,  and  thus  to  assist  the  currents 
in  carrying  them  away. 

An  instructive  account  of  the  formation  of  anchor  ice  in 
one  of  the  rivers  of  New  Brunswick,  during  the  winter  of 
1869-70,  has  been  recorded  by  W.  G.  Thompson,'  one  of 
the  engineers  of  the  Intercolonial  Railway.  In  this  ac- 
count, quoted  below,  it  is  stated  that  not  only  were  small 
stones  Hfted  from  the  bottom  and  floated  down-stream, 
but,  what  is  of  s^ill  greater  interest,  the  ice  increased  in 
thickness  in  some  instances  until  it  formed  dams,  and  the 
stream  was  turned  from  its  course.  Mr.  Thompson's  ac- 
count is  as  follows: 

■■■■■■■"■■*---■■  ■  \     ■ " 

"  The  Matapediac,  which  is  fed  by  large  fresh-water  springs, 
runs  over  a  rocky  bottom  covered  with  loose  stones,  ranging  in 

^Nature,  vol.  i.,  p.  555,  1870. 


i|ii^«r-« 


26 


R/VERS  OF  NORTH  AMERICA 


size  from  coarse  gravel  to  boulders  as  large  as  a  hogshead,  and 
the  average  currer^t  is  about  four  miles  an  hour. 

"  Early  in  November  last  the  temperature  went  down  in  one 
night  to  12°  F.,  and  on  going  out  of  camp  the  following  morning 
I  noticed  large  quantities  of  what  appeared  to  be  snow  saturated 
with  water  heating  down  the  stream,  but  not  a  particle  of  snow 
had  fallen  near  us  for  many  miles  round,  as  far  as  I  could  see 
by  the  mountain-tops,  nor  had  any  ice  formed  on  the  surface  of 
the  river, 

"  The  water  opposite  where  I  stood  was  about  six  feet  deep, 
.tud  perfectly  clear,  so  that  1  could  see  every  stone  on  the  bottom, 
and,  with  the  exception  of  the  floating  slush,  the  river  was  as  it  had 
been  the  previous  day  when  the  temperature  was  about  50°  F.  I 
got  into  a  canoe  and  paddled  with  the  current  for  half  a  mile  or 
so,  and  in  shooting  some  small  rapids,  where  the  water  in  places 
was  not  more  than  two  or  three  feet  deep,  I  noticed  on  the 
bottom  m  .sses  of  the  slush  clustered  round  and  between  the 
boulders,  and  a  slight  touch  with  the  paddle  was  sufficient  to  free 
these  clusters,  when  they  rose  to  the  surface,  and  were  carried 
away  by  the  current.  I  continued  down  the  stream  for  three  or 
four  miles,  and  noticed  the  same  thing  in  every  rapid,  where  the 
water  was  shallow  and  ruffled  by  stones  at  the  bottom. 

"  The  buoyancy  of  this  slush  was  such  that  when  detached 
from  the  bottom  it  rose  so  rapidly  as  to  force  itself  well  out  of 
the  water,  and  then  floated  off  about  half  submerged. 

"  I  watched  this  forming  of  slush  for  many  days,  and  in  several 
cases  found  small  stones  embedded  in  the  floating  slush,  having 
been  torn  from  the  bottom  when  the  buoyancy  of  the  slush,  aided 
by  the  running  water,  caused  it  to  rise. 

"  The  temperature  continued  getting  lower  daily,  and  the  slush 
in  the  rapids  formed  more  rapidly  than  it  was  carried  away,  so 
much  so  that  a  bar  or  dam  was  formed  across  the  river  at  each 
rapid,  backing  up  the  water  in  some  cases  five  or  six  feet,  when 
it  generally  found  an  outlet  over  the  a  .joining  land,  and  into  its 
natural  bed  again,  or  the  head  of  water  became  suflticient  to  tear 
away  the  obstruction,  which  by  this  time  had  become  a  solid 
frozen  mass. 


LAfVS  GOVERNING  THE   STREAMS 


27 


"  All  this  time,  no  properly  crystallised  ice  had  formed  on  the 
surface  of  the  river,  the  current  being  too  rapid,  but  the  slush  of 
'  anchor  ice,'  as  the  trappers  call  it,  was  forming  in  deeper 
water  than  it  had  formed  in  before,  indeed  all  over  the  river 
bottom,  and  was  rising  and  floating  away  as  I  have  already  de- 
scribed. Eventually  the  temperature  got  down  to  two  and  three 
degrees  below  zero,  when  the  river  surface  began  to  freeze  in  the 
eddies  and  along  the  edges,  and  the  open-water  space  became 
narrower  every  day,  and  was  filled  with  floating  '  anchor  ice  ' 
and  detached  masses  of  solid  ice,  which  here  and  there  became 
jammed  and  frozen  together,  so  as  to  form  ice-bridges  on  which 
we  could  cross. 

"  These  ice-bridges  served  as  booms  to  stop  much  of  the  float- 
ing ice,  which  froze  solid  the  moment  it  came  to  rest;  and  in  this 
manner  the  river  at  last  became  completely  frozen  over  for  about 
forty  miles  of  its  length,  but  not  until  after  we  had  experienced 
five  weeks  of  steady  cold,  with  the  thermometer  never  above  12" 
F.,  and  frequently  down  to  —  16"  F." 

When  we  recall  the  fact  that  the  conditions  of  tem- 
perature described  above  recur  every  winter  throughout 
nearly  one-half  of  North  America,  it  becomes  evident  that 
anchor  ice  must  play  an  important  part  in  stream  trans- 
portation. But  little  attention  has  been  directed  to  this 
matter,  however,  and  it  is  highly  desirable  that  someone 
favourably  located  for  such  studies  should  make  a  careful 
record,  especially  as  to  the  number  and  size  of  the  stones 
picked  up  from  the  bottom  of  stream  channels,  owing  to 
the  buoyancy  of  the  ice  formed  about  them.  It  will  be 
noted  that  this  process  of  transportation  is  brought  into 
operation  simply  by  a  lowering  of  temperature,  and  does 
not  require  a  rise  of  the  water  in  order  to  float  the  debris 
attached  to  the  ice,  as  is  the  case  when  surface  ice  becomes 
fastened  to  the  bottom.     Anchor  ice  operates  in  the  way 


^ 


*'  "'  I 
«  -  •    J 


.3 


28 


RIVERS  OF  NORTH  AMERICA 


described  in  midstream,  where  the  water  is  not  only  swift 
but  may  be  comparatively  deep;  while  the  similar  work  of 
surface  ice  is  practically  confined  to  ',he  shallow  water  on 
the  margins  of  streams. 

Corrasion. — Clear  streams,  as  we  ordinarily  see  them,  are 
such  as  have  removed  from  their  channels  all  of  the  particles 
within  their  reach  which  they  are  competent  to  transport, 
although  they  may  still  roll  and  push  coarse  fragments  along 
their  bottoms.  It  is  to  be  noted,  however,  that  clear 
streams,  when  of  a  given  velocity,  may  become  muddy  if  the 
velocity  is  increased.  The  friction  of  clear  running  water 
is  but  slight,  hence  streams  not  charged  with  material  in 
suspension  wear  their  channels  very  slowly.  In  such  in- 
stances chemical  solution  of  the  rocks  over  which  the  waters 
flow  may  be  in  excess  of  mechanical  abrasion. 

When  a  stream  receives  an  initial  load  of  silt  and  sand 
from  rain-wash,  the  action  of  the  wind,  glacial  abrasion, 
etc.,  an  important  change  in  its  behaviour  occurs.  The 
transported  fragments  on  being  brought  in  contact  with  the 
bottom  and  sides  of  the  channel  of  the  stream  produce 
abrasion.  Thr  flowing  waters  charged  with  silt  and  sand 
act  not  unlike  a  strip  of  sandpaper  that  is  drawn  over  an 
object  continually  in  one  direction.  The  transported  frag- 
ments abrade  the  rocks  with  which  they  are  brought  in 
contact,  and  ar""  themselves  worn  and  broken.  Gravel  and 
larger  rock  fiagments  too  heavy  to  be  carried  in  suspension, 
except  during  floods,  are  worn  and  broken  by  smaller  frag- 
ments coming  In  contact  with  them,  and  when  moved  during 
high-water  stages,  assist  in  a  marked  way  in  promoting  the 
process  of  channel  enlargement.     The  friction  and  impact 


LAWS  GOVERNING  THE   STREAMS 


29 


of  the  particles  that  are  carried  forward  tend  to  loosen  and 
dislodge  other  fragments,  and  thus  increase  the  amount  of 
material  available  for  transportation. 

It  is  convenient  to  consider  the  process  of  stream  abrasion  ' 
due  to  the  friction  of  transported  material  and  to  chemical 
solution,  as  a  part  of  the  general  process  of  land  degradation, 
and  to  give  it  a  separate  name.     To  meet  this  want,  the 
term  corrasion  has  been  proposed.' 

The  deepening  and  widening  of  a  stream  channel — that  is, 
corrasion — is  carried  on  mainh-  by  mechanical  wear  due  to 
the  friction  of  silt,  sand,  gravel,  boulders,  etc.,  carried 
through  it  by  the  flowing  waters,  but  is  assisted,  many 
times  in  an  important  manner,  by  solution. 

There  are  conditions  that  limit  or  modify  the  competency 
of  a  stream  to  transport  debris,  as  has  already  been  briefly 
considered.  In  a  similar  way  there  are  conditions  which 
modify  and  limit  the  rate  of  stream  corrasion.  Streams,  as 
is  well  known,  vary  in  rate  of  flow,  in  volume,  in  declivity, 
in  the  degree  to  which  they  are  loaded,  in  chemical  com- 
position, etc.  Changes  in  any  one  of  these  conditions  will 
manifestly  exert  an  influence  on  the  rate  at  which  a  stream 
is  enabled  to  deepen  and  widen  its  channel. 

The  nature  of  the  load  carried  by  a  stream  also  varies  in 
different  instances,   and  even   from    month  to  month  and 

'  As  the  nomenclature  of  dynamical  Kcology  and  physical  geography  is  not 
yet  definitely  fixed,  it  may  be  suggested  that  corraaion  furnishes  a  convenient 
generic  term,  and  may  be  made  to  include  the  jiroccsses  of  abrasion  by  stienm- 
like  movements  of  other  substances  than  water,  when  charged  with  rock 
fragments.  The  grinding  of  rocKs  by  glaciers  may  be  designated  as  glacial 
corrasion  ;  the  process  of  wearing  of  rocks  by  dust  and  sand  transported  by  air- 
currents  becomes  aoHan  cortasion  ;  and  lake  and  ocean  shores  furnish  examples 
of  wave  and  current  corrasion. 


■    ■   ■^ii'      ■'•' 

K.I  »■ 

if./ 


^^^SmSSSSfWiom 


30 


RIVERS  OF  NORTH  AMERICA 


from  day  to  day  in  the  same  stream.  The  particles  or  frag- 
ments carried  are  fine  or  coarse,  hard  or  soft,  rounded  or 
angular;  all  of  these  conditions  have  an  influence  on  the 
amount  of  friction  exerted  on  the  stream  bed,  and  hence 
modify  the  rate  of  corrasion.  Again,  the  rate  at  which  a 
stream  channel  is  enlarged  under  the  supposition  that  the 
velocity  of  the  stream,  the  character  of  its  load,  etc.,  remain 
constant,  will  vary  with  the  nature  of  the  rocks  over  which 
it  flows.  Hard  rocks  are  worn  more  slowly  than  soft  rocks 
easily  soluble  rocks  are  more  rapidly  removed  than  those  of 
difficult  solubility.  There  are  still  other  conditions  pertain- 
ing to  the  beds  of  streams  which  influence  corrasion.  Mas- 
sive rocks  yield  less  readily  than  those  perhaps  of  equal 
hardness  and  equally  soluble,  but  which  are  much  jointed,  or 
occur  in  thin  layers.  Rock  texture  thus  exerts  an  import- 
ant influence  on  corrasion,  as  does  also  the  inclination  of 
the  rocks  or  their  dip.  When  hard  and  soft  beds  alternate, 
other  conditions  being  the  same,  corrasion  is  more  rapid 
when  they  are  inclined  than  when  they  are  horizontal. 

It  is  unnecessary  to  trace  the  effects  of  variations  in  these 
several  conditions,  as  the  student  may  do  this  for  himself, 
and  thus  have  the  pleasure  of  making  independent  dis- 
coveries. In  the  case  of  a  stream  flowing  under  stated  con- 
ditions, let  the  student  postulate  an  increase  in  volume,  in 
declivity,  in  character  of  load,  in  hardness  of  the  rocks 
forming  its  channel,  etc.,  other  conditions  remaining  the 
same  in  each  case,  except  so  far  as  the  reaction  on  them  of 
the  postulated  change  is  concerned,  and  trace  the  effects  on 
the  rate  of  corrasion. 
The  principal  laws  governing  corrasion  are,  briefly: 


ZAPVS  GOVERNING  THE   STREAMS 


31 


1.  The  rate  at  which  a  stream  corrades  its  channel,  other 
conditions  remaining  the  same,  increases  with  increase  in 
load  to  a  certain  point  which  varies  with  the  character  of 
the  load.  If  the  load  continues  to  increase,  friction  on  the 
bottom  decreases,  and  ceases  when  the  entire  energy  of  the 
stream  is  consumed  in  transportation. 

2.  Other  conditions  remaining  the  same,  corrasion  in- 
creases with  declivity  and  with  volume  of  water,  since  each 
of  the  changes  increases  velocity. 

For  a  more  detailed  discussion  of  the  laws  governing 
stream  transportation  and  corrasion  than  it  is  practicable  to 
present  at  this  time,  the  reader  is  referred  to  Gilbert's  '  ad- 
mirable analysis  of  land  sculpture,  already  cited  many  times 
in  the  present  treatise. 

The  conditions  controlling  the  amount  of  detritus  a  stream 
can  transport  are  mainly  velocity  and  volume.  Velocity  is 
increased  by  an  increase  in  volume  and  also  by  increased 
declivity.  In  nature  we  find  that  streams  ordinarily  vary 
in  volume  with  seasonal  changes  and  also  from  day  to 
day,  and  hence  their  ability  to  transport,  and  consequently 
to  corrade,  undergoes  many  fluctuations.  The  gradients 
of  stream  channels  vary  from  place  to  place  along  their 
courses,  and  hence  their  ability  to  deepen  their  channels  is 
not  the  same  in  all  parts.  The  load  that  a  stream  carries  in 
one  portion  of  its  course  may  be  too  great  a  burden  in 
another  portion,  and  some  of  it,  always  the  coarser  portion, 
will  be  dropped.  The  journeys  of  stream-borne  debris  are 
thus  far  from  being  continuous.     The  transported  material 

'  G.  K.  Ciilbert,  Report  on  the  Geology  of  the  Henry  Mountains,  4to,  pp.  99- 
150.  Department  of  the  Interior,  U.  S.  (leographical  and  (Jeological  Survey 
of  the  Rocky  Mountain  Region,  J.  W.  Powell  in  charge.     Washington,  1877. 


f 

t 


3* 


RIVERS  OF  NORTH  AMERICA 


is  laid  aside  from  time  to  time  in  bars  and  flood-plains.  It 
may  require  tens  of  thousands  of  years  for  a  given  rock 
fragment  loosened  on  a  mountain-side  to  reach  its  final 
resting-place  in  the  sea. 

In  nature  we  find,  as  a  rule,  that  the  gradients  of  streams 
decrease  from  their  sources  to  their  mouths,  but  it  must  be 
remembered  that  this  is  the  result  of  the  action  of  the 
streams  themselves  and  follows  a  long  period  of  development 
and  adjustment.  As  increased  declivity  favours  corrasion, 
it  is  to  be  expected  that  the  mountain  tracts  of  streams 
will  be  deepened  at  a  greater  rate  than  their  valley  tracts, 
and  that  they  will  be  enabled  to  extend  their  branches 
farther  and  farther,  and  thus  acquire  new  territory.  This, 
in  fact,  is  the  case,  as  we  kriow,  for  the  great  majority  of 
rivers  are  corrading  their  channels  in  the  highlands  and  de- 
positing in  the  lowlands.  Rivers  are  ordinarily  supplied  by 
many  branches,  however,  which  means  that  the  volume  of 
water  in  the  branches  is  less  than  in  the  trunk  stream,  and 
accompanying  decreased  volume,  other  conditions  remain- 
ing the  same,  is  a  decreased  corrasion.  The  behaviour  of 
a  stream  in  reference  to  corrasion  and  deposition  is  thus  a 
resultant  of  many  and  frequently  opposing  conditions.  As 
will  be  shown  later,  the  ability  of  a  stream  to  corrade  or  de- 
posit in  a  given  portion  of  its  course  varies  ordinarily  with 
its  age,  or,  more  accurately,  with  its  stage  of  development. 
The  portion  of  a  stream  channel  where  corrasion  is  in  active 
progress  during  its  youth,  may  become  a  region  of  deposi- 
tion at  a  more  advanced  stage  in  its  history  under  the  pro- 
cess of  normal  development  which  streams  experience  even 
if  no  changes  occur  in  land  elevation.  


LA^S  GOVERNIXG  THE    STREAMS 


33 


Pot-Holes. — One  of  the  minor  phases  of  stream  corrasion  is 
illustrated  by  the  cylindrical  holes  frequently  worn  in  the 
beds  of  streams  by  stones  swept  about  by  strong  currents. 
These  holes  are  sometimes  saucer-shaped,  but  more  fre- 
quently have  steep  sides  and  rounded  bottoms,  and  resemble 
the  insides  of  the  familiar  cast-iron  kettles  used  for  culinary 
purposes ;  this  similarity  has  suggested  the  name  pot-hole,  by 
which  they  are  commonly  designated.  Their  walls  are 
usually  smooth,  and  sometimes  exhibit  grooves  and  ridges 


m 


f 


Fig.  I.     A  Pot-Hole  being  Scoured  out  by  .Stream  Action. 
(After  R.  S.  Tarr.) 

in  horizontal  planes  or  arranged  more  or  less  spirally.  In 
these  grooved  holes  one  sometimes  finds  well-worn  pebbles, 
or  even  large  boulders,  and  discovers  a  relation  between  the 
size  of  the  grooves  and  of  the  stones  that  made  them.  In 
many  instances,  also,  these  mills  are  still  in  working  order, 
and  a  stream  of  water  is  plunging  into  them,  as  is  shown  in 
the  accompanying  photograph. 


*s£ 


r 

In*'  ■^. 


34 


RIVERS  OF  NORTH  AMERICA 


Pot-holes  are  of  all  sizes,  from  shallow  depressions  a  few 
inches  in  depth  to  vertical  borings  five  or  six  feet  across  and 
fifteen  or  twenty  feet  or  more  deep.  From  these  well  de- 
fined examples  there  is  a  gradation  up  to  the  barins  pro- 
duced by  the  stones  swirled  about  at  the  bases  of  waterfalls, 
as  the  pool  into  which  Niagara  plunges,  for  example,  which 
might  perhaps  be  termed  compound  pot-holes. 

In  the  making  of  these  characteristic  depressions  the  mill- 
stones may  be  worn  out,  but  new  ones  are  supplied  from 
time  to  time,  and  the  process  goes  on.  Similar  excavations 
are  made  also  beneath  glaciers,  where  streams  flowing  on 
the  surface  of  the  ice  plunge  into  crevasses,  or  deep  weil- 
like openings  termed  moulins,  and  reach  the  rocks  below. 
In  fact,  any  strong  current  by  being  deflected  may  cause 
loose  stones  to  be  whirled  about  so  as  to  grind  the  rocks  on 
which  they  rest,  and  produce  depressions  of  the  natur(;  here 
considered.  Favourable  conditions  result  when  pebbles 
and  boulders  of  hard  material  occur  in  a  stream  where  the 
bottom  is  of  soft  rock,  and  where  also  the  current  is  swift, 
and  eddies,  swirls,  whirlpools,  etc.,  are  produced. 

Lateral  Corrasion. — Streams  in  moving  material  along 
their  channels  not  only  wear  away  the  rocks  over  which 
they  flow,  but  abrade  the  sides  of  their  channels  as  well. 

It  is  difficult  to  formulate  the  laws  governing  lateral  cor- 
rasion, but  the  general  manner  in  which  it  is  accomplished 
may  be  readily  understood. 

In  considering  the  process  of  vertical  corrasion  the  influ- 
ence of  upward  currents  in  flowing  water  w  s  recognised. 
But  besides  the  upward  currents  there  are  also  lateral  cur- 
rents.    In  fact,  the  flow  of  water,  even  through  smooth. 


straig 
areg€ 
the  d 
flowinj 
many  ; 
moving 
channe 
against 
pension 
particle; 
The  lar^ 
can  take 
widening 
in  susper 
the  cours 
for  any  ci 
cave  and 
against  oi 
fleeted, 
At  the  Io( 
bank,  th( 
stream's 
tlown-strcc 
the  concav 
so  easily  ti 
Hence   th( 
latter. 

Lateral 
checked  by 
feasons. 


lAlVS  GOVERNING  THE   STREAMS 


^i 


straight  troughs,  is  complex,  and  many  secondary  currents 
are  generated.     This  complexity  is  vastly  increased  when 
the  channel  is  rough  and  irregular.     If  we  watch  a  swift- 
flowing  stream,  we  shall  be  enabled  to  see  that  there  are 
many  swirls  and  eddies  due  to,  or  accompanied  by,  currents 
moving   in   all   directions.     Along  the   sides  of   a  stream 
channel  the  secondary  currents  strike  the  rocks  and  dash 
against  them  whatever  material  the  waters  may  hold  in  sus- 
pension.    Friction  results  from  the  impact  of  the  floating 
particles,  and  tends  to  wear  away  the  sides  of  the  channel. 
The  larger  fragments  rolled  along  the  bottom  of  the  stream 
can  take  but  little  part,  directly,  in  this  process  of  channel- 
widening.     It  is  the  finer  material — the  silt  and  sand  held 
in  suspension — which  does  most  of  the  work.     Moreover, 
the  courses  of  stream  channels  are  seldom,  if  ever,  straight 
for  any  considerable  distance,  but  are  a  succession  of  con- 
cave and  convex  curves.     The  water  is  alternately  thrown 
against  one  bank,  and  the  direction  of  the  current  being  de- 
flected, impinges  on  the  opposite  bank  lower  down-stream. 
At  the  locality  where  the  thread  of  swiftest  flow  nears  the 
bank,  the  rocks  are  worn  away,  and  the  irregularity  of  the 
stream's  course  increased.     The  material  removed  is  carried 
down-streaii.,  and  in  part  deposited  in  the  slack  water  on 
the  concave  side  of  the  next  bend.     Swift  streams  are  not 
so  easily  turned  aside  as  those  which  flow  less  impetuously. 
Hence  the   former   maintain  straighter    courses   than  the 
latter. 

Lateral  corrasion  may  go  on  when  vertical  corrasion  is 
checked  by  decrease  in  declivity,  deposition,  or  for  other 
reasons.     After  a  stream  has  cut  down  its  channel  at  its 


tr 


RIVERS  OF  NORTH  AMERICA 


mouth  to  the  level  of  the  still  water  into  which  it  discharges 
and  established  a  low  gradient,  lateral  corrasion  may  con- 
tinue. It  is  under  these  conditions  that  the  widening  of 
river  valleys  is  mainly  accomplished.  In  general,  vertical  is 
so  far  in  excess  of  lateral  corrasion  in  the  case  of  high-grade 
streams,  that  the  valleys  produced  are  narrow,  and  V-shaped 
in  cross-section,  while  under  similar  conditions  i.i  respect  to 
climate,  rock  texture,  etc.,  in  a  low-grade  stream,  although 
its  actual  rate  of  lateral  corrasion  may  be  less  than  in  the  first 
instance  cited,  the  ratio  r^i  lateral  to  vertical  corrasion  is 
greater,  and  a  flat-bottomed  valley  results.  The  cross  pro- 
file of  a  valley  widened  by  lateral  corrasion  is  U-shaped,  and 
if  the  process  is  long  continued  becomes  broad-bottomed. 
The  statement  frequently  made  that  a  stream-cut  valley  is 
V-shaped  in  cross-profile,  in  distinction  from  the  U-shape 
of  valleys  formerly  occupied  by  glaciers,  is  not  strictly  true, 
as  it  considers  only  young  stream-valleys. 
r  Meandering  Streams. — The  serpentine  courses  followed 
'  especially  by  sluggish  streams,  just  referred  to,  is  a  matter 
of  more  than  passing  interest  to  the  student  of  geography. 
Much  of  the  charmingly  picturesque  which  enters  into  and 
many  times  forms  the  leading  feature  of  stream-side  scenery, 
as  well  as  the  secret  of  the  process  by  which  valleys  are 
broadened,  and  the  adjacent  uplands  removed,  results  from 
the  meandering  and  lateral  migration  of  streams.  By  thi;; 
same  general  process,  too,  as  will  be  considered  later,  the 
flood-plains  of  rivers  are  spread  out.  Illustrations  of  the 
curves  characteristic  of  many  streams,  are  given  on  Plate  3, 
The  causes  leading  to  the  meandering  of  streams  have 
been  studied  by  various  observers.     As  stated  by  Fergus- 


son,' 

vvhos 

plane 

botto 

flow  ( 

howe\ 

consid 

'ty,   h( 

action 

the  osc 

tween  1 

straighi 

to  ^WQ 

In  natu 
and  the 
bears  a ( 
of  their 


Th 


e  s 


cu 


broad 

common 

ducing 

which   1 

then  flov 

rest  to  w 

current  a 

of  old 

abandon 
raised  fro 

'James  Fe 

T>'(  Quarter 

1S63. 


a 
el 


LAPVS  GOVERNING  THE   STREAMS 


n 


.il. 


son/  a  river  is  a  body  of  water  in  unstable  equilibriurnT) 
whose  normal  condition  is  that  of  motion  down  an  inclined 
plane,   and  if  all  inequalities  in  the   material  forming  the; 
bottom  and  sides  of  its  channel  could  be  removed,  it  would) 
flow  continuously  in  a  straight  line.      (It  is  to  be  noted, 
however,  that  the  influence  of  the  earth's  rotation  is  not 
considered  in  this  discussion.)     Any  obstruction  or  inequal- 
ity,  however,   necessarily  induces  an  oscillation,   and,   the 
action  being  continuous,   the  effects   are  cumulative,   and 
the  oscillation  goes  on  increasing  till  it  reaches  a  mean  be- ' 
tween  the  force  of  gravity  tending  to  direct  the  current  in  a 
straight  line,  and  the  force  due  to  the  obstinction  tending 
to  give  a  direction  more  or  less  at  right  angles  to  the  former.  ; 
In  nature  not  one  but  many  disturbing  conditions  occur, 
and  the  streams  flow  in  a  series  of  curves,  each  of  which  i 
bears  a  definite  relation  to  their  volumes  and  the  gradients  * 
of  their  channels. 

The  stage  in  the  'ives  of  rivers  when  they  meander  in 
broad  curves  through  rich  bottom-lands,  is  usually,  or  most 
commonly,  reached  late  in  their  lives,  when  the  task  of  re- 
ducing their  channels  to  the  level  of  the  still  water  into 
which  they  discharge  has  been  nearly  completed.  They 
then  flow  sluggishly,  and  may  be  said  to  be  enjoying  the 
rest  to  which  a  long  life  of  activi<-y  entitles  them.  A  slack 
current  and  a  tortuous  course  are  not  infallible  indications 
of  old  age,  however.  Young  streams  flowing  across  an 
abandoned  lake  bed,  for  example,  or  over  lands  recently 
raised  from  the   sea,   may  have    these   characteristics.     A 

'James  F'ergusson,  "On  Recent  Changes  in  the  Delta  of  the  Ganges,"  in 
The  Quarterly  ypurnal  of  the  Geological  Society  of  London,  vol.  xix.,  p.  323, 


I 

Ik.    * 

"^■iw 


38 


mVEKS  OF  NORTH  AMERICA 


winding  course  may  be  retained  by  a  river  which  has  been 
given  renewed  energy  by  a  re-elevation  of  the  land,  even 
after  it  has  cut  a  deep  trench  and  is  a  hurrying  torrent. 
The  tendency  to  meander  is  strongest  in  streams  that  are 
heavily  loaded  and  are  depositing  a  portion  of  their  burdens 
in  flood-plains. 

Although  the  tendency  to  meander  characterises  all 
streams,  for  the  reason  that  their  channels  are  never 
straight  or  composed  of  homogeneous  material  for  any  con- 
siderable distance,  yet  the  process  carries  with  it  certain 
limitations,  as  will  be  shown  in  discussing  the  origin  and 
nature  of  flood-plains  and  terraces  in  a  subsequent  chapter. 

Other  Curves. — Not  only  do  streams  bend  to  the  right 
and  left  of  their  general  courses,  as  where  a  river  meanders 
through  a  broad,  partially  alluvial-filled  valley,  but,  as  will 
be  described  later,  form  curves  in  a  vertical  plane  as  well. 
Where  corrasion  is  in  progress,  the  longitudinal  profile  of 
the  channel  produce(  s  concave  to  the  sky,  and  where  de- 
position occurs,  curves  convex  upward  result.  More  or  less 
complete  spiral  curves  about  vertical  or  inclined  axes,  like 
the  twists  of  a  corkscrew,  may  be  seen  when  a  high-grade 
stream  is  excavating  soft  clays,  and  where  a  brook  on  the 
surface  of  a  glacier  plunges  into  a  well-like  opening  in  the 
ice.  The  influence  of  the  graceful  sweep  of  stream-curves, 
on  the  beauty  of  many  landscapes,  is  due  to  their  infinite 
variety;  no  two  in  the  course  of  even  a  great  river  being 
identical.  This  marvellous  diversity,  produced  by  simple 
means,  becomes  still  more  impressive  when  it  is  remembered 
that  no  one  of  these  many  curves  remains  the  same  for  any 
considerable  period  of  time.  : _   _  _  _ 


r- 


Plate  ill. 


F^o.  A.     Ray  Brook,  Adirondacks.  New  York 
(Photograph  by  S.  R.  Stoddard.) 


'ijdse* 


•*'■*•« 


^^^^^:  ^  ;/;'r^iij  * 


"^■;°^"^"''st---t--— .. 


The 

are  su 

in  the 

gracefi 

swirls, 

and  ar 

but  th( 

line,  al 

the  cui 

strong 

a  convi 

flowing 

in  level 

instance 

be  two  ( 

the  surf 

upward 

a  down\ 

gentle,  I 

part  but 

Driftvv 

as  will  b 

the  eleva 

tendency 

rising;  w 

slack  wat 

in  mid-st 

probably 

l^cflccti 

The  earth 


LAfrS  GOVERNING  THE   STREAMS 


39 


The  several  classes  of  curves  in  the  channels  of  streams 
are  supplemented  in  an  interesting  manner  by  other  curves 
in  the  surfaces  of  the  streams  themselves.  Not  only  are 
graceful  curves  produced  by  the  flow  of  water  in  eddies  and 
swirls,  or  when  they  arch  over  or  circle  about  obstacles, 
and  are  thrown  into  waves  by  the  wind  and  other  causes, 
but  the  surface  of  a  stream  in  cross-section  is  not  a  straight 
line,  although  this  condition  is  very  nearly  reached  when 
the  current  is  gentle,  and  the  waters  deep.  If  there  is  a 
strong  central  current,  however,  the  surface  there  forms 
a  convex  curve,  which  rises  well  above  the  more  gently 
flowing  waters  on  either  side.  In  swift  rivers  this  difference 
in  level  frequently  amounts  to  five  or  six  feet,  and  in  certain 
instances,  as  at  the  whirlpool  below  Niagara,  is  reported  to 
be  two  or  three  times  these  measures.  In  such  examples 
the  surface  line  of  a  cross-profile  wou'<d  show  a  pronounced 
upward  curve  in  the  central  part,  bordered  on  each  side  by 
a  downward  curve.  The  bordering  downward  curves  are 
gentle,  but  may  be  recognised,  although  they  probably  de- 
part but  little  from  a  straight  line. 

Driftwood  carried  by  a  stream  with  a  swift  central  current, 
as  will  be  describea  more  fully  in  advance,  tends  to  leave 
the  elevated  central  part  and  collect  along  the  banks.  This 
tendency  may  be  seen  especially  when  swift  streams  are 
rising;  when  the  waters  fall,  however,  driftwood  leaves  the 
slack  water  adjacent  to  the  shore,  and  tends  to  concentrate 
in  mid-stream.  At  such  times  the  stream  in  cross-profile 
probably  presents  a  concave  surfacr-line. 

Deflection  of  Streams  Owing  to  the  Rotation  of  the  Earth, — 
The  earth,  as  is  well  known,  makes  one  rotation  from  west 


%V-..    3 


40 


B I  VERS  OF  NORTH  AMERICA 


to  east  about  an  axis  passing  through  the  poles,  in  23  hours 
\  56  minutes  and  4  seconds.  The  circumference  of  the  earth 
!  being  about  24,000  miles,  any  point  on  the  equator  must, 
therefore,  travel  over  1000  miles  an  hour.  North  and  south 
of  the  equator  this  motion  gradually  decreases,  and  becomes 
zero  at  the  poles.  This  motion  has  an  influence  on  the  f^ow 
of  streams,  and  tends  to  cause  them  to  follow  curved  instead 
of  straight  courses.  This  may  be  most  readily  understood 
i  in  the  case  of  streams  having  either  north  or  south  direc- 
1  tions,  but  affects  all  streams  on  the  earth's  surface,  unless 
!  they  follow  strictly  the  path  marked  out  by  the  equator. 
Water  flowing  northward  from  the  equator  would  start 
with  the  motion  from  west  to  east,  which  pertains  to  that 
location,  but  as  it  advanced  would  cross  regions  which 
have  progressively  less  and  less  motion  from  west  to 
east.  The  current  due  to  gravity,  we  will  assume,  tends 
due  north,  but  the  waters  have  also  a  motion  from  west  to 
east,  due  to  the  earth's  rotation,  which  is  in  excess  of  the 
similar  motion  of  the  region  necessarily  invaded.  The  re- 
sultant of  these  two  forces  will  carry  the  stream  to  the  east 
of  the  meridian  on  which  it  started,  and  the  stream  will  curve 
to  the  right  of  its  initial  course.  In  a  similar  way,  a  stream 
in  the  northern  hemisphere,  flowing  toward  the  equator, 
would  be  continually  invading  territory  having  greater  and 
greater  motion  from  west  to  east  and  would  curve  to  the 
west  of  the  path  it  would  follow  if  influenced  only  by  gravity. 
Thus,  in  the  northern  hemisphere,  the  tendency  of  the 
earth's  rotation  is  to  cause  the  streams,  no  matter  what 
their  direction  of  flow,  to  corrade  their  right  more  than 
their  left  banks.      In  the  southern  hemisphere  the  direction 


of  ci 

strea 

their 

tion 

in  an 

force 

right 

hemis^ 

The 

it  inva 

inotioi 

same  I 

mumc 

course, 

(lirectic 

fest  in 

greater 

this  ten 

the  righ 

^f  their  \ 

the  earti 

than  the 

initial  co 

operative 

the  bank; 

difficult  tc 

'  William  I 
Surface,"  in 
"^  "le  earths 
/:/. mnttary  X 


^^•ys  GOVE,^m..a  THE  STREAMS 


of  curvature  due  to  the  earth's  rotation  •  '  ^' 

streams,   no  matter  what  th.     !,  "  '^''"^^'  «"<'  'he 

'heir  ieft  more  than  their  righ    bar"";;  '"'  '°  '""^^'^ 
fion  tostreamsof  Ferrel',,  ^''.s  ,s  an  applic-,- 

force  arisi,„fro,n  ..e  LV^rTr"'  ''""  "  "  ''^"""-^ 

hemisphere. '"  ^        '    ''^  ^^  ^^'^  ^^/^  in  the  southern 

The  tendency  of  a  stream  f 
"  'nvades  territory  hav^ra  oT"'"'""  '  ■''"'■^'"  ^"^^  - 

■"ofon  is  greater  the  grealr  th      7"""^  '''""^''"8  ^'^  °f 
•.lit  greater  the  velorif-^r  r^(  <.t 

-me  ,s  true  of  the  parts  of  a  streak  Th      ,     ''"'"'  ^  '"^ 

mum  current  in  a  s.reamfoII„    •  ^""^  thread  of  maxi- 

— .s  .n  the  ce:  r^:  :::r;r-'--'y-a%ht 

ciirection  owing  to  rotation'     th      ,  """  ^''""^^  ■" 

'«t  in  the  thread  of  mlxim  '  '  '"'  ''"'■^'">'  ■"«"■- 

»'"ggi^h  waters  on  cithers^^and  T^:'  "'^"  ''"  '"^  -- 
greater  deflection.      Where  ,tr'  .        "''"'"''  ""d^rgoes 

""■'  tendency  leads  to  a„7„  •  '°"°"  "''"'""8  -""'ses 

'heright.i„.he„orttrn    rmMl""  "'^'  "■^^"^-"'^■^  - 
«'  their  general  direcon      t'T'^^''^'^;  ""^^  "'an  to  the  left 

'he  earth-,  rotation,  for  ihem  tT  "    '""  "  ''"''"'y'  "»"=  '"1 

'han  their     .ft  banks  ,^Z  ""''  '"^''^  "K"'  '"-e 

'•""'a'  cours,    .     Thi  'ten.  '""^  '°  ""  """'"  ■"  'heir 

"Petative.     Owing,"  Uet":!  ''  '"''l'  ^^  ="'  'he  time 

'he  banks  of  streams  Id  oil      T"    '""  '"  ''-^"e.ssof  I 

^;«-'  'o  discover  e^  ,:'r:t:tj;;"''':'°"-^' "  ■•■' 


• 

t    ' 


II" 


43 


RIVERS  OF  NORTH  AMERICA 


plainly  controlled  their  migrations.  An  illustration  of  such 
a  result  is  thought  to  be  furnished,  however,  by  the  streams 
on  the  south  side  of  Long  Island,  where  there  is  a  plain 
with  a  remarkably  even  descent  and  gentle  slope.  This 
plain  is  crossed  by  a  number  of  small  streams  which  have 
excavated  shallow  valleys  in  essentially  homogeneous 
gravel.  Each  of  these  little  valleys  is  bordered  on  the 
west,  or  right  side,  by  a  bluff  from  ten  to  twenty  feet  high, 
while  its  gentle  slope  on  the  left  side  merges  imperceptibly 
with  the  general  plain.  The  stream  in  each  case  follows 
closely  the  bluff  at  the  right.  As  stated  by  Elias  Lewis, 
and  affirmed  by  Gilbert,'  there  seems  to  be  no  room  for 
reasonable  doubt  that  these  peculiar  features  result  from 
the  influence  of  terrestrial  rotation. 

It  is  to  be  remembered  that  the  force  of  rotation,  like 
gravity,  is  all  the  time  operative,  but  its  influence  is  greatest 
on  streams  flowing  north  or  south,  is  greater  in  high  than 
in  low  latitudes,  and  increases  with  the  rapidity  with  which 
the  waters  arc  transferred  from  an  area  having  a  certain 
motion  to  another  area  having  a  different  motion.  The 
results  of  this  influence,  although  not  conspicuous,  are  nev- 
ertheless important.  There  is  a  slight  tendency  through- 
out the  length  of  every  stream  in  America  and  at  all  times, 
to  erode  its  right  more  rapidly  than  its  left  bank.  In  the 
case  of  the   Mississippi,   shown    by  Gilbert  in  the  article 

'  G.  K.  Gilbert,  "  The  Sufficiency  of  Terrestrial  Rotation  for  the  Defection 
of  Streams,"  in  American  Journal  of  Sn'encf,  vol.  xxvii.,  pp.  437-432,  3d 
series,  1884.  An  abstract  of  this  paper,  accompanied  by  an  extension  of  the 
discussion,  may  be  found  in  Stifucf,  vol.  iv.,  pp.  28,  2(),  1884.  See,  .also,  \V. 
M.  Davis,  "  An  Early  Statement  of  the  Deflective  ESect  of  the  Earth's  Kota- 
Hon."  in  Sciencr,  vol.  i.,  p.  98,  1883.   — — .. 


cite 

the 

the 

Q< 
ing 

leadi 
accoi 
WJ 
clear 
forme 
Great 
sedim( 
small 
but  m( 
amcun 
and  bn 
ter.     T 
power  t 
tlie  fraj 
to  mov< 
gone  or 
ffeograp 
itself,  bi 
The  n 
souri  anc 
'Jy  Ioad< 
'"  high  I 
"lany  bm 
heavy  ]o, 
^'ntcring  t 


LAH^S  GOVERNING  THE   STREAMS 


43 


cited  above,  the  selective  tendency  thus  determined  toward 
the  right  bank  is  nearly  nine  per  cent,  greater  than  toward 
the  left  bank. 

Questioning  the  Rivers. — To  illustrate  the  laws  govern- 
ing the  behaviour  of  streams,  let  us  see  how  lome  of  the 
leading  features  of  the  rivers  of  North  America  can  be 
accounted  for. 

Why,  for  example,  are  the  waters  of  the  St.  Lawrence) 
clear   and    those  of  the    Missouri    usually    muddy  ?      The  \ 

former  is  obviously  a  clear  stream  for  the  reason  that  the  , 

1 

Great  Lakes  it  drains  act  as  settling  basins  and  retain  the  i 

i 

sediment   brought    down    by  countless  tributaries.     Many  j 
small  streams  join  the  St.  Lawrence  below  Lake  Ontario, 
but  most  of  these  also  have  lakes  in  their  courses,  and  the 
amount  of  the  sediment  reaching  the  main  river  from  rills 
and  brooks  is  not  sufficient  to  materially  change  its  charac- 
ter.    The  clear  waters  of  the  St.  Lawrence  have  but  little  ,' 
power  to  corrade.     The  current  is  swift  in  places,  but  all  of 
the  fragments  in  its  bed  which  the  current  is  competent  i 
to  move  have  long  since  been  carried  away.     Corrasion  has  , 
gone  on  with  extreme   slowness   throughout   the   present  ' 
geographical  cycle,   and  the  river  has  not  yet  entrenched 
itself,  but  is  practically  a  surface  stream. 

The  reader  will,  no  doubt,  at  once  remark  that  the  Mis- 
souri and  the  IMatte  are  also  surface  streams,  although  heav- 
ily loaded  with  silt  and  sand.  These  rivers,  however,  rise 
in  high  mountains  and  flow  across  a  broad  plateau.  Their 
many  branches  in  the  mountains  are  swift  and  bear  along 
heavy  loads  of  detritus.  On  leaving  the  mountains  and 
entering  their  plain  tracts,  velocity  is  checked,  and  the  less 


mmmmmmmmmm 


m 


44 


RIVERS  OF  NORTH  AMERICA 


swift  waters  are  no  longer  able  to  carry  the  loads  they  pre- 
viously transported  with  ease,  and  deposition  occurs.  Then, 
too,  the  surfaces  of  the  Great  Plateaus  are  composed  of 
easily  eroded  rocks.  During  every  rain  quantities  of  soil, 
etc.,  are  washed  into  the  rivers,  and  during  the  long,  dry 
summers,  the  winds  are  busy  in  performing  a  similar  task. 
At  present  not  only  is  corrasion  nil  throughout  the  plain 
tracts  of  the  Missouri  and  the  Platte,  but  sedimentation  is 
in  progress.  These  rivers  are  aggrading  previously  eroded 
valleys. 

The  question  of  how  deep  and  how  wide  a  river  valley 
shall  be,  depends  not  alone  on  the  elevation  of  the  land 
above  sea-level,  but  also  on  the  ratio  of  stream  corrasion 
to  the  general  waste  or  erosion  of  the  bordering  lands. 
When  the  rate  of  stream  corrasion  is  in  marked  excess  of 
the  rate  at  which  the  general  surface  of  the  land  is  being 
eroded,  deep,  narrow  stream  channels  result.  But  if  the 
erosion  progresses  at  nearly  the  same  rate  as  corrasion,  the 
relief  of  the  region  will  be  mild.  Between  these  two  ex- 
tremes there  are  many  intermediate  stages.  Overloaded 
streams,  by  dropping  a  portion  of  their  burdens,  may 
not  onlv  spread  out  broad  flood-plains,  but  also  elevate 
their  channels  so  as  to  flow  at  a  higher  level  than  the  sur- 
rounding land.  The  Platte  and  the  Missouri  are  now  de- 
positing material  throughout  their  courses  across  the  Great 
Plateaus,  and,  as  just  stated,  are  aggrading  previously 
formed  broad-bottomed  valleys. 

Many  conditions  besides  those  just  noticed  exert  an  influ- 
ence on  the  character  and  expression  of  stream-cut  valleys. 
When  the  rocks  are  hard,  they  tend  to  form  precipitous 


LAWS  GOVERNING  THE   STREAMS 


45 


bluffs;  when  soft,  they  crumble,  and  the  valleys  have  flaring 
sides.  When  the  climate  is  arid,  the  wasting  away  of  cliffs 
is  long  delayed,  and  if  corrasion  is  actively  progressing, 
steep-sided  gorges,  or  canyons,  result.  Vegetation  retards 
surface  erosion,  although  favouring  rock  decay,  and  has  a 
varied  influence  on  the  lives  and  character  of  the  streams. 
These  and  still  other  influences  modify  the  results  of  stream 
corrasion,  which  find  expression  in  the  scenery  of  the  land. 
It  may  be  asked,  why  is  it  that  the  Colorado  has  carved 
the  most  magnificent  canyon  in  North  America,  while  the 
Platte,  rising  in  the  same  mountains,  is  bordered  throughout 
much  of  its  course  by  perhaps  the  least  picturesque  scenery 
of  any  large  river  on  the  continent  ?  Much  of  the  answer  has 
already  been  given.  The  Platte,  as  we  have  seen,  is  aggrad- 
ing its  channel  to  the  east  of  the  Rocky  Mountains.  The 
Colorado  is  still  corrading  from  its  source,  with  the  excep- 
tion of  certain  slack-water  reaches  in  soft  rocks,  all  the  way 
to  its  place  of  discharge.  The  rocks  through  which  the 
Platte  flows  are  soft ;  while  those  cut  by  the  Colorado  are 
hard.  The  region  of  the  High  Plateaus  crossed  by  the 
Colorado  has  experienced  a  somewhat  recent  uplift  of  several 
thousand  feet ;  while  the  country  traversed  by  the  Platte  to 
the  east  of  the  Rocky  Mountains  has,  so  far  as  is  known, 
undergone  but  slight  changes  in  elevation  during  the  same 
period.  The  climate  of  the  Colorado  region  is  arid,  and 
surface  waste  from  rains  and  the  action  of  the  wind  probably 
less  than  in  the  region  of  the  Platte.  It  is  in  these  and  per- 
haps still  other  contrasts  of  conditions  that  the  striking 
differences  in  the  scenery  along  the  border  of  these  two 
rivers  may  be  accounted  for. 


^^ 


0. 


m 
r 
• 
r 

SI. 

»«^ 

1  ■;■  ■' 


mm 


46 


RIVERS  OF  NORTH  AMERICA 


By  comparing  the  scenery  of  various  other  regions  and 
seeking  for  the  underlying  causes  to  which  their  differences 
are  due,  the  student  may  arrive  at  a  juster  appreciation  of 
the  chara».Leristics  of  river  work  than  a  formal  statement  of 
the  subject  will  furnish. 

Erosion. — Weathering,  transportation,  and  corrasion  are 
three  agencies  which  by  their  combined  action  lead  to  the 
removal  of  land  upraised  above  the  sea,  and  the  production 
of  a  vast  array  of  ever-changing  topographic  forms.  To 
this  far-reaching  and  highly  complex  process,  the  name 
erosion  has  been  given. 

Briefly  stated,  the  removal  of  material  from  land  areas  is 
accomplished  by:  ist.  The  disintegration  of  the  rocks  by 
both  mechanical  and  chemical  means,  through  the  action  of 
the  varied  and  complex  process  termed  weathering.  2d. 
The  removal  especially  of  the  finer  products  produced  by 
weathering,  by  the  wind,  general  rain-wash,  and  rills,  and  of 
both  fine  and  coarse  debris  by  brooks  and  rivers,  by  a  pro- 
cess termed  transportation.  3d.  The  friction  and  impact 
of  the  material  transported,  accompanied  by  solution,  lead 
to  the  deepening  and  broadening  of  stream  channels,  or 
corrasion. 

A  necessary  accompaniment  of  erosion  is  the  deposition 
of  the  material  removed,  or  sedimentation.  A  temporary 
phase  of  sedimentation  is  the  laying  aside  of  stream-carried 
detritus  in  flood-plains  and  stream  channels,  but  its  final 
resting-place,  so  far  as  a  single  geographical  cycle  is  in- 
volved, is  on  the  floor  of  the  sea. 

Basclevel  of  Erosion. — The  depth  to  which  a  stream,  flow- 
ing into  a  lake  or  the  sea,  can  lower  its  channel  by  mechani- 


ZAtyS  GOVERNING  THE   STREAMS 


47 


cal  corrasion  is  limited  by  the  surface  level  of  the  receiving 
water-body.  As  mechanical  corrasion  decreases  in  more 
than  a  simple  ratio  with  decrease  in  declivity,  the  final 
stages  in  the  lowering  of  a  stream  channel  to  the  level  of 
the  still  water  into  which  it  flows  must  be  extremely  slow. 
Corrasion  is  not  limited  to  mechanical  processes,  however, 
but  includes  solution  as  well.  The  final  reduction  of  a 
stream  channel  to  sea-level  must,  therefore,  be  by  solution. 
Every  stream,  when  its  entire  history  is  reviewed,  will  be 
found  to  be  engaged  in  deepening  its  channel  to  the  horizon 
referred  to,  or  else  has  accomplished  a  part  or  the  whole  of 
the  task.  The  datum-plane  limiting  downward  corrasion  is 
reached  first  at  the  mouth  of  a  stream  and  is  then  continued 
progressively  towards  its  source.  When  many  streams  are 
considered,  various  stages  in  this  process  may  be  recognised. 
The  depths  to  which  streams  may  excavate  their  channels 
is  evidently  a  matter  of  great  importance  in  their  develop- 
ment and  in  the  history  of  the  topographic  changes  of  the 
land.  This  fundamental  principle  was  first  clearly  defined 
by  Powell,'  who  employed  the  term  basclcvcl  to  designate 
the  lower  limit  of  stream  action.  It  is  the  baselevel  of  cor- 
rasion. A  lake  determines  the  baselevel  for  the  streams 
flowing  into  it,  but  as  lakes  are  short-lived,  the  real  base- 
level  toward  which  all  streams  are  working  is  the  surface 
level  of  the  sea. 

To  use  Powell's  own  words  in  this  connection : 

"  We  may  consider  the  level  of  the  sea  to  be  a  grand  base- 
level,  below  which  the  dry  land  cannot  be  eroded;  but  we  may 

'  J.  W.   Powell,   Exploration  of  the  Colorado   River  of  the    West  and  its 
Tributaries^  p.  203,  410.     Woahiugton,  D.  C,  1875. 


48 


HrVERS  OF  NORTH  AMERICA 


also  have,  for  local  and  temporary  purposes,  other  baselevels  of 
erosion,  which  are  the  levels  of  the  beds  of  the  principal  streams 
which  carry  away  the  products  of  erosion.  I  take  some  liberty 
in  using  the  term  level  in  this  connec:tion,  as  the  action  of  a  run- 
ning stream  in  wearing  its  channel  ceases,  for  all  practical  pur- 
poses, before  its  bed  has  cjuite  reached  the  level  of  the  lower  end 
of  the  stream.  What  1  have  called  the  baselevel  would,  in  fact, 
be  an  imaginary  surface,  inclining  slightly  in  all  its  parts  toward 
the  lower  end  of  the  principal  stream  draining  the  area  through 
which  the  level  is  supposed  to  extend,  or  having  the  inclina- 
tion of  its  path  raised  in  direction  as  determined  by  tributary 
streams. 


When  a  stream  has  lowered  its  channel  nearly  to  base- 
level,  downward  corrasi'  is  retarded,  but  lateral  corrasion 
continues.  Low-grade  sircams,  as  we  have  seen,  arc  the 
ones  most  inclined  to  meander,  and  to  broaden  their  valleys. 
If  this  process  is  continued  for  a  sufficient  time  in  any 
region,  it  will  lead  to  the  removal  of  all  land  within  reach  of 
the  streams,  down  to  their  own  level.  Baselevel  of  corrasion 
thus  becomes  practically  the  baselevel  of  erosion.  The  ulti- 
mate result  of  erosion  is  to  reduce  a  land  area  to  a  plain  at 
sea-level.  Such  perfect  plains,  however,  arc  exceedingly 
rare,  but  approximations  to  the  ultimate  result  are  common, 
and  plains  in  this  penultimate  stage  have  been  named  pctn- 
plains  by  Davis. 

A  peneplain  is  the  normal  result  of  the  erosion  of  the 
land,  provided  elevation  or  depression  do  not  occur  to 
check  the  process.  If  a  portion  of  the  earth  crust  remains 
essentially  stable  for  a  sufficient  length  of  time  to  allow  the 
streams  flowing  from  it  to  broaden  their  charnels,  a  more 
or  less  extensive  peneplain  is  produced.     Such  periods  of 


LAIVS  GOVERNING  THE   STREAMS 


49 


stability,  when  movements  in  the  earth's  crust  are  not  suffi- 
cient to  check  the  normal  process  of  baselevelling,  arc  termed 
geographical  cycles.  A  peneplain  may  be  defined  as  an  ap- 
proximately perfect  plain  produced  by  erosion  during  a 
geographical  cycle.  If,  after  a  portion  of  a  land  area  has 
been  reduced  to  a  peneplain,  elevation  occurs,  a  new  geo- 
graphical cycle  will  be  initiated,  and  the  process  of  base- 
levelling  again  begun. 

During  one  geographical  cycle  all  of  the  land  may  not  be 
reduced  to  a  plain,  but  isolated  uplands  between  broad 
valleys  remain.  These  remnants  are  left  as  an  inheritance 
to  the  next  succeeding  geographical  cycle.  Mount  Monad- 
nock,  in  southern  New  Hampshire,  is  an  example  of  such  a 
remnant,  and  forms  a  promiiicnt  feature  on  the  surface  of 
an  elevated  peneplain.  The  study  of  the  relief  of  the  land 
in  various  regions  has  shown  that  there  are  many  such  rem- 
nants, left  by  incomplete  planation.  A  name  is  needed 
for  such  topographic  features,  and  to  meet  this  want  Davis 
has  proposed  that  they  be  termed  monadnocks,  after  the 
typical  example  just  referred  to.  A  monadnock,  then,  is  a 
hill  or  mountain  left  standing  on  a  peneplain,  owing  to 
incomplete  planation. 

The  laws,  just  stated,  governing  the  reduction  of  lajid 
areas  to  baselevcl,  although  wide-reaching  and  fundamental 
to  the  student  of  geography,  are  not  strictly  true,  or,  rather, 
exceptions  to  them  may  be  found.  The  downward  limit  of 
mechanical  corrasion  is  not  in  all  cases  the  sea-level.  Gla- 
ciers may  enter  the  sea  and  continue  their  destructive  work 
at  a  depth  of  a  few  hundred  feet  below  its  surface.  Ice- 
bergs may  also  disturb  the  bottom  of  the  sea  at  considerable 


-jtr- 


m  M 


SO 


A'/r/:A'S  OF  NORTH  AMERICA 


depths.  Currents  in  the  sea  sometimes  corrade  the  bot- 
tom ;  the  downward  limit  to  which  this  process  may  be 
carried  is  unknown,  but  may  be  hundreds  of  fathoms.  The 
removal  of  rocks  in  solution  may  be  carried  en  deep  below 
sea-level;  the  lower  limit  has  not  been  determined,  but  is 
certainly  many  thousands  of  feet.  Change  in  the  position 
of  rock  material  through  the  agency  of  plants  and  animal.s 
is  not  limited  downward,  by  the  surface  level  of  the  sea,  but 
goes  on  below  that  horizon.  All  of  these  processes,  how- 
ever, are  of  minor  importance,  and  need  not  be  considered 
as  sensibly  modifying  the  conclusion  that  the  downward 
limit  to  which  land  areas  may  be  reduced  is  the  horizon  of 
the  surface  of  the  sea.  Strictly  sper.king,  baselevel  is  the 
lower  limit  of  the  mechanical  corrasion  of  streams,  but  prac- 
tically, as  we  say,  it  is  also  the  downward  limit  of  erosion. 

lujiucncc  of  Vegetation  on  Erosion. — The  influence  of 
vegetation  on  the  general  process  of  denudation  is  varied, 
and  both  retards  and  accelerates  the  process.  Vegetation 
breaks  the  force  with  which  rain-drops  strike  the  earth,  and 
besides,  when  the  ground  is  covered  with  leaves,  or  with 
grasses,  moss,  or  other  plants  of  low  growth,  the  surface 
waters  are  filtered  of  such  debris  as  they  may  have  taken 
in  suspension.  Vegetation  thus  retards  transportation  and 
decreases  mechanical  corrasion.  On  the  other  hand,  vegeta- 
tion furnishes  the  percolating  water  with  organic  acids, 
principally  humus  acids,  which  greatly  enhance  their  solvent 
power.  Hence  vegetation  favours  chemical  corrasion  in  a 
high  degree. 

The  roots  of  plants  bind  the  soil  together,  and  thus  assist 
it  in  resisting  mechanical  agencies  tending  to  remove  it. 


LAiVS  GOVERNING  THE   STREAMS 


51 


But  roots  furnish  or^.iuic  acids  as  they  decay,  and  besides 
open  passaj^eways  for  descending  water,  thus  facilitating 
chemical  changes.  The  student  may  readily  observe  other 
modifications  in  the  lives  of  streams  due  to  vegetation  and 
to  climate.  Interesting  results  would  no  doubt  be  obtained 
from  the  study  of  the  vegetation  which  grows  in  the  streams 
themselves,  such  as  the  alg.x  and  certain  higher  forms  of 
plant  life.  The  direct  influence  of  driftwood  and  of  fallen 
timber  is  considered  in  a  subsequent  chapter. 

Although  the  summary  of  the  laws  governing  the  be- 
haviour and  work  of  streams  just  presented  is  confessedly 
incomplete,  yet  it  is  the  writer's  hope  that  it  will  serve  to 
interest  the  reader  in  the  processes  of  land  sculpture  nearly 
everywhere  in  progress  where  the  earth's  surface  rises  above 
the  sea,  and  suggest  questions  to  which  more  technical 
treatises,  or,  better  still,  the  rills  and  rivers  themselves  will 
furnish  answers. 

Note. — Since  this  chapter  was  written,  a  highly  instructive  pay)er  by  Hunt- 
ington Hooker  has  appeared,  on  "  The  Suspension  of  Solids  in  Flowing 
Water,"  Transactions  of  the  American  Society  of  Civil  Engineers,  vol.  xxxvi., 
1897,  PP-  239-340,  which  the  reader  is  recommended  to  study. 


Hi     t 


CHAPTER  111 

INFLUENCE  OF  INEQUALITIES  IN    THE  HARD- 
NESS OF  ROCKS  ON  RIVER-SIDE  SCENERY 


IF  \vc  watch  a  hillside  rill  which  is  born  durin^r  a  heavy 
shower  ami  runs  dr\-  when  the  sun  aijain  shines,  it  will 
be  noted  that  in  the  steeper  portion  of  its  descent  it  cuts  a 
narrow  trench  and  deposits  much  of  the  material  removed 
farther  down  its  course.  It  soon  becomes  apparent  that  the 
little  stream  is  corradin^  where  its  descent  is  steep,  and 
raising  its  bed  by  depositing  the  material  brought  from 
above  where  tlie  grade  becomes  gentle.  Tl:e  processes  of 
corrasion,  transportation,  and  deposition  may  all  be  seen  in 
operation  in  a  stream  only  a  few  rods  in  length.  The  topo- 
graphic forms  resulting  are,  on  .  minute  scale,  the  same  as 
those  which  give  grandeur  to  many  far-rcacliing  views  of 
river-side  scenery. 

The  process  of  excavation  and  deposition  carried  on  by 
a  rill  leads  to  the  making  of  a  uniform  grade  down  which 
the  waters  continue  to  carry  debris.  This  gradient,  as  will 
be  shown  later  in  discussing  the  profiles  of  streams,  is  not 
an  inclined  plane,  but  in  longitudinal  profile  is  a  curve, 
steepes*^  near  the  source  of  the  stream,  and  flattening  out 
and  approaching  nearer  and  nearer  a  .straight  line  the  nearer 

5a 


INEQUALITIES  IN  THE    HARDNESS  OE  A'OCA'S 


53 


the  mouth  of  the  rill  is  approachcil.  The  immediate  task 
of  the  rill  may  be  said  to  be  the  makiii};  of  a  certain  {gradient, 
which  is  best  suited  to  its  volume  and  other  conditions. 

In  the  portion  of  the  rill  channel  where  corrasion  is  in 
progress,  the  waters  are  broken  by  little  cascades  and  mini- 
ature rapids,  and  it  is  evident  that  a  uniform  gradient  in 
that  portion  of  its  bed  ;  far  from  perfect.  The  reason  is 
that  the  clay,  earth,  or  other  material  in  which  the  rill  is 
working  is  of  varying  tlegrees  of  hardness.  When  a  boulder 
is  crossed,  a  cascade  results.  When  the  material  is  soft  the 
channel  is  quickh  deepened.  Each  passage  from  a  hard  to 
a  soft  layer  is  marked  by  a  cascade.  Evidently  the  charac- 
ter of  the  material  which  the  rill  is  removing  exerts  a  con- 
trolling inflviencc  i)n  the  miniature  scenery  along  all  of  its 
upper  course. 

If  we  enlarge  our  field  of  observation,  similar  characteris- 
tics will  be  found  in  many  brooks,  creeks,  and  rivers.  The 
study  of  many  streams  will  soon  show,  however,  that  some 
of  them  are  broken  by  cascades  and  rapids,  while  others, 
except  on  their  extreme  headwaters,  have  even  descents 
and  flow  with  a  generally  uniform  current.  Those  which 
are  broken  by  cascades,  we  soon  learn,  for  the  most  part  oc- 
cupy narrow,  steep-sided  trenches,  and  are  apt  to  have  lakes 
along  their  courses;  while  those  with  a  uniform  descent  from 
near  their  sources  to  their  mouths,  are  not  associated  with 
lakes,  except  perhaps  such  as  are  formed  by  the  streams 
themselves  in  alluvial-filled  valleys. 

A  comparison  of  many  streams  will  show  that  the  differ- 
ences just  referred  to,  depend  on  their  age,  or,  more  p^cur- 
atcly,    on    the   stage   of  development   they  have  reached. 


54 


RIVERS  OF  NORTH  AMERICA 


Young  streams,  or  such  as  have  not  cut  down  their  channels 
so  as  to  produce  a  uniform  gradient,  are  the  ones  suppHed 
in  part  by  lakes,  and  are  broken  by  places  of  rapid  descent ; 
while  older  streams  have  removed  the  inequalities  from  heir 
channels,  and  the  lakes  that  may  formerly  have  existed  2\'  '  ^ 
their  courses  have  been  drained.  Il  thus  becomes  evident 
that  the  degree  to  which  variations  in  the  hardness  of  the 
rocks  influjnce  the  scenery  of  streams  is  greatest  in  youth 
and  gradually  beomes  less  and  k-ss,  but  seldom,  if  ever, 
entirely  vanishes. 

All  stages  in  development,  from  extreme  youth  to  slug- 
gish old  age,  may  be  recognised  in  the  streams  of  North 
America,  and  among  the  most  ma.  ced  characteristics  of  this 
slow  change  is  the  presence  of  cataracts  and  rapids  in  the 
courses  of  the  streams  wImcIi  have  not  yet  made  a  decided 
advance  in  their  appointc  i  tasks. 

Waterfalls. — Yo  .ng  streams  are  obliged  to  accept  in- 
herited conditions  of  slope,  and  may  discover  that  their 
courses  are  broken  by  places  of  deep  descent,  and  rapids 
ind  cascacies  result.  If.  for  example,  a  stream  originates 
on  a  tableland,  with  an  irregular  surface,  or  is  perhaps 
bounded  by  an  escarpment,  it  will  h»\'e  an  un<»vo*»  channel, 
at  least  for  a  \\\\\k\,  and  be  broken  by  places  of  steep  descent. 
Again,  stvodius  coming  Into  existence  on  the  withdrawal  of 
an  ice  sheet,  or  the  iltiilulng  of  u  lake  will  usually  find  in 
equalities  in  theh  channel'  which  will  cause  waterfalls.  In 
such  ln**it\nies  \\\k\  i(>!\(litlnMs  producing  the  falls  are  an  in- 
heritantu  from  pre-existing  topographic  conditions.  As 
stream  development  progresses,  jjowever,  the  cascades  re 
suiting  from  inherited  conditions  disappear;  but  others  due 


INEQUALITIES  IN  THE   HARDNESS  OF  ROCKS 


55 


icl, 

hit. 

of 

in- 

In 

in- 


to inequalities  in  rock  texture,  to  the  more  rapid  rates  at 
which    a   trunk   stream   may  deepen   its    channel  than   its 
branches,   to   the   loads   sometimes   deposited    in    sluggish 
streams   by    swifter 
tributaries,    and    stili 
other  causes,   make 
their  appearance. 

Iii  illustration  of 
the  processes  by 
which  cascades  origi- 
nate t|}t*iiigh  the  ac- 
tion of  the  streams 
themselves,  it  is  evi- 
dent  that  when  a 
stream  flows  across 
alternating  hard  and 
soft  beds,  the  soft 
beds  will  be  removed 
more  easily  than 
those  of  greater  resist- 
ance, and  when  the 
streams  leave  a  hard 
bed,  a  rapid,  cascade, 

or   waterfall    may    be      I'l,..  -.     a  \ouinj  Valley  ht-inn  cut  m  shale 
produced.     The  refer-     Ro^-k,  Central  New  York.    (After  R.  S.   larr.) 

ences  just  made  to  a  trunk  stream  cutting  more  rapidly 
than  its  tributaries,  and  the  deposition  of  debris  in  a  .sluggish 
stream  at  the  mouth  of  a  high-grade  tributary,  need  no 
special  explanation. 

Ail  of  the  conditions  just  referred  to,  including  inherited 


ii- 


56 


RIVEJ^S  OF  NORTH  AMERICA 


inequalities  of  channel  and  the  production  of  places  of  steep 
descent  during  stream  development,  pertain  principally  to 
young  streams.  As  a  stream  advances  in  its  task  of  cutting 
down  to  baselevel  and  acquires  the  gradient  best  adapted 
to  its  work,  inequalities  in  the  slope  of  its  channel  disap- 
pear. Cataracts,  of  whatever  character,  are  usually  an  index 
of  immature  stream  development.  The  development  of  a 
stream  progresses,  however,  from  its  mouth  towards  its 
source,  and  the  feeding  brooks  of  even  w^ell-developed  river 
systems  are  young,  and  may  have  cascades,  while  the  lower 
portions  of  the  same  drainage  system  may  have  reached 
perfect  adjustment.  The  streams  draining  the  southern 
Appalachians  have  been  allowed  to  progress  with  the  execu- 
tion of  their  tasks  without  serious  interruption  for  a  long 
period  of  time,  all  lakes  and  waterfalls  resulting  from  in- 
herited conditions,  and  practically  all  cascades  produced  by 
irregularities  in  rock  texture,  have  long  since  disappeared 
throughout  their  lower  courses,  but  their  head  branches  are 
still  young  and  are  yet  being  extended.  On  these  young 
twigs  of  the  drainage  system  cascades  are  common.  An 
illustration  of  a  cascade  of  this  nature  is  presented  in 
Plate  IV. 

An  exception  to  the  rule  that  cascades  are  not  developed 
in  the  courses  of  mature  streams,  is  so^r.?^ilnes  found  in  lime- 
stone regions  where  surface  drainage  enters  underground 
channels.  The  breaking  of  the  roof  of  a  cavern  may  lead  to 
the  production  of  a  ''ascade  at  any  stage  in  the  life  of  a 
stream,  but  the  chances  of  such  an  accident,  as  it  may  be 
termed,  become  less  and  hss  as  baselevel  co:  J.i  c /^  :'*i  ap- 
proached. 


INEQUALITIES  IN  THE   HARDNESS  OF  ROCKS 


57 


In  addition  to  the  causes  juFt  considered,  there  are 
changes  produced  by  movements  in  the  earth's  crust,  as 
when  the  rocks  are  folded  or  faulted ;  the  birth  and  growth 
of  glaciers;  '.olcanic  eruptions;  the  deposition  of  rock  ma- 
terial from  solution,  as  when  a  spring  precipitates  traver- 
tine, siliceous  sinter,  etc.,  in  the  course  of  a  stream;  the 
work  of  animals,  as  when  beavers  build  their  dams;  the 
stranding  of  driftwood  so  as  to  block  drainage,  etc.,  which 
may  interrupt  the  even  flow  of  water,  and  give  origin  to 
rapids,  cascades,  and  waterfalls. 

The  tens  of  thousands  of  waterfalls  in  North  America 
may  be  arranged  principally  in  two  classes :  those  resulting 
from  the  excavation  of  alternating  hard  and  soft  layers,  and 
occurring  mostly  on  the  head  branches  of  well-developed 
streams,  as  in  the  southern  Appalachians;  and  those  due  to 
previous  glacial  conditions.  Of  these  two  classes  the  second 
is  by  far  the  more  numerous,  and  furnishes  the  most  magnifi- 
cent examples  of  waterfall  of  all  grades. 

As  already  stated,  the  northern  half  of  North  America 
was  formerly  covered  by  ice-sheets.  Local  centres  of  snow 
accumulation  and  of  glaci  i/s  also  existed  on  the  Rocky  and 
Cascade  Mountains,  and  the  Sierra  Nevada,  far  to  the  south 
of  the  southern  limit  of  the  former  continental  glaciers. 
Throughout  nearly  all  of  the  vast  regions  which  were  form- 
erly ice-covered,  the  melting  of  the  glaciers  left  the  land 
encumbered  with  debris.  The  surface  inherited  by  the  post- 
glacial streams  was  essentially  a  new-land  area,  and  the 
streams  have  not  progressed  far  enough  in  their  develop- 
ment to  have  removed  the  inequalities  in  their  channel."-, 
and  waterfalls  arc  commoh. 


-'.tk*>- 


t.  '.■•' 


^     ,.Mt 


9i- 


58 


RIVERS  OF  NORTH  AMERICA 


Illustrations  of  this  class  of  cascades  are  furnished  by 
those  in  the  picturesque  streams  of  the  Catskills,  Trenton 
Falls,  the  many  beautiful  cataracts  near  Ithaca,  the  well- 
known  instances  in  Watkins  Glen,  and  numberless  others 
of  the  same  general  character  in  New  York.  Still  more 
numerous  instances  might  be  cited  in  Canada,  as,  for 
example,  the  leap  made  by  the  water  at  the  Falls  of  Mont- 
morenci,  near  Quebec,  the  Great  Falls  in  Labrador.  In 
fact,  scarcely  a  stream  can  be  ascended  in  all  of  the  eastern 
and  northern  portion  of  th'  formerly  glacier-covered  region, 
without  discoveiing  that  it  has  recently  been  turned  from 
its  former  channel,  or  exhibits  the  cliaracteristics  of  youth 
from  mouth  to  source. 

On  the  headwater  of  the  Mississippi,  and  about  the  upper 
Great  Lakes,  the  drift  sheet  is  thicker  than  farther  eastward, 
and  the  streams  have  less  frequently  cut  down  to  the  hard 
rocks  beneath,  so  as  to  develop  cascades.  It  is  only  the 
stronger  rivers  in  this  more  thoroughly  drift-covered  country 
that  have  progressed  far  enough  with  their  recently  added 
task,  to  lay  bare  the  solid  rock  beneath  the  superficial 
covering. 

A  far-reaching  result  of  the  disturbance  produced  in 
stream  development  by  the  glacial  epoch  is  seen  in  the  dis- 
tribution of  w  iter  power  pnuluced  by  it.  In  New  England, 
manufacturing  industries  werts  Hoon  eptabllshed  after  the 
coming  of  Europeans,  and  a  decided  ittiprtssion  made  on 
the  character  of  the  people  by  this  circumstance.  South  of 
the  glacial  b<Hjndary.  water  po«%r  was  far  less  abundant, 
and  mostly  within  tlw  more  inaccessible  portions  of  the 
mountains,    the    development  of  manufacturing  industries 


INEQUALITIES  IN  THE  HARDNESS  OF  ROCKS 


59 


was  hence  delayed,  and  attention  given  more  largely  to 
agriculture,  for  which  climatic  and  other  conditions  were 
more  favourable.  When  steam  was  introduced  as  a  motive 
power,  the  waterfalls  at  the  north  declined  in  importance  as 
sources  of  energy,  but  in  this  budding  age  of  electricity  they 
are  again  coming  into  demand. 

In  all  of  the  various  phases  of  stream  development  and  of 
the  diversity  in  the  relief  of  the  land  produced  by  stream 
erosion,  thus  far  considered,  a  marked  feature  has  been  that 
changes  are  continually  in  progress.  To  this  rule,  the  water- 
falls furnish  no  exception.  They  have  their  periods  of 
growth  and  decline,  and  in  many  instances  shift  their 
positions,  or  migrate. 

In  the  case  of  waterfalls  resulting  from  inherited  topo- 
graphic conditions,  they  may  spring  into  existence  all  at 
once,  and  at  the  very  start  be  grander  than  ever  after. 
Niagara,  when  it  first  leaped  from  the  summit  of  the  escarp- 
ment near  the  present  site  of  Lewiston,  was  higher  than  at 
any  subsequent  period  of  its  history.' 

When  falls  result  from  the  wearing  away  of  soft  rocks  so 
as  to  make  adjacent  hard  beds  prominent,  there  is  usually 
iit  first  a  small  difference  in  relief,  producing  a  rapid,  and 
then  greater  changes  resulting  in  a  cascaue  the  height  of 
which  is  increased  by  reason  of  the  increased  energy  of  the 
waters  as  they  plunge  into  the  pool  below,  but  there  comes 
a  time  when  the  stream  channel  below  the  fall  can  be  low- 
ered no  farther.  The  cascade,  or  waterfall,  as  we  may 
choose  to  call  it,  then  reaches  its  greatest  development,  or 

'G.  K.  Gilbert,  "  Niagara  Falls  and  their  History, "  in  National  GrograpUt 
Mono^rafihs,  vol.  i. ,  pp.  21)3-236.     Ainctican  Book  Co.,  1895. 


fi 


6o 


RIVERS  OF  NORTH  AMERICA 


its  majority.  But  the  lowering  of  the  stream  channel  above 
the  fall  continues,  and  its  height  gradually  decreases.  Such 
a  sequence  in  the  life  of  a  waterfall  originating  from  unequal 
stream  corrasion  in  soft  and  hard  rocks,  would  result  when 
the  fall  did  not  migrate  up  stream  but  remained  in  one 
place.  But  most  waterfalls  are  subject  to  a  process  of 
migration.  .. 

The  Migration  of  Waterfalls. — When  a  hard  layer  causing 
a  waterfall  is  horizontal,  or  but  slightly  inclined,  the  escarp- 
ment over  which  the  waters  plunge  recedes,  owing,  in  most 
instances,  to  the  removal  of  softer  rocks  beneath,  by  the 
friction  of  stones  washed  about  by  the  swirling  waters,  and 
the  fall  migrates  up  stream,  leaving  a  more  or  less  canyon- 
like valley  to  mark  the  path  along 
which  it  travelled.  Thus,  below  Ni- 
agara Falls  there  is  a  canyon  about 
seven  miles  long,  and  approximately 
two  hundred  feet  deep,  which  has 
been  left  by  the  migration  of  the 
cataract. 

If  the  hard  bed  cut  through  by  a 
stream  so  as  to  produce  a  cascade  has 
a  sharp  downward  slope  in  the  direc- 
tion opposite  to  the  flow  of  the  stream, 
the  fall  will  become  lower  and  lower 
as  corrasion  progresses,  and  when  the 
hard  layer  is  passed,  the  life  of  the 
cataract  wiil  come  to  an  end.  If, 
however,  as  may  happen,  the  hard  layer  dips  downstream, 
the  rapid  or  fall  produced  will  manifestly  increase  in  magni- 


Fl'^.  3.  Profile  and  Sec- 
tion at  Middle  of  Horse- 
shoe  Fall,  Niagara, 
Showing  Hard  Lime- 
stone above  Soft  Shale, 
and  Probable  Depth  of 
the  Pool  into  which  the 
Waters  Plunge.  Scale  : 
I  inch  =  384  feet.  (After 
G.  K.  Gilbert.) 


fH     1 


Plate  IV. 


,:,m.^i!: 


Fig.  a.     Fall  on  Black  Creek  near  Gadsden,  Alabama. 

A  young  branch  of  Coosa  River,  at  the  south  end  of  Lookout  '.viountain  ;  hard  sandstone 

above  shale. 


S»  l£m> 


1* 


II 

I- 


"IV 


I.' 


Fig.  B.     Echo  River  in  Mammoth  Cav;,  Kentucky. 
(Copyrighted  photograph  by  H.  C.  Ganter.) 


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INEQUALITIES  IN  THE  HARDNESS  OF  ROCKS         6l 


tude  until  the  locality  wh  re  the  outcropping  edge  of  the 
hard  bed  comes  to  the  surface  is  cut  through,  and  then  a 
comparatively  sudden  lowering  and  adjustment  of  grade 
will  follow.  When  the  hard  layer  which  a  stream  has  to  cut 
through  is  horizontal  instead  of  being  inclined  either  with 
or  against  the  current,  it  presents  a  greater  task  to  a  stream 
cutting  through  it  than  in  any  othci  position,  because  the 
mass  of  rocks  necessary  for  the  stream  to  remove  in  order 
to  reduce  the  grade  of  its  channel  is  greater  than  if  the 
bed  is  inclined.  When  the  rocks  are  horizontal,  the  life  of 
a  cataract  may  be  immensely  prolonged. 

The  only  cases  in  which  waterfalls  produced  by  hard  ad- 
jacent to  soft  rocks  do  not  migrate,  are  when  the  hard  layer 
is  vertical.  In  such  a  case  the  change  in  the  position  is 
limited  to  the  thickness  of  the  resistant  bed.  In  the  Cas- 
cade Mountains  there  are  numerous  rapids  and  cascades  due 
to  vertical  dikes  of  basalt.  These  dikes  are  harder  than 
the  adjacent  rock,  and  cause  inequalities  in  the  beds  of 
the  streams  crossing  them,  but  the  positions  of  the  falls 
produced  remain  essentially  the  same  throughout  their  lives. 

In  the  streams  flowing  eastward  from  the  Appalachians, 
falls  and  rapids  occur  where  they  leave  the  hard  crystalline 
rocks  forming  the  Piedmont  Plateau  and  enter  the  soft  rocks 
of  the  Coastal  Plain.  As  these  falls  recede  up  stream,  they 
leave  canyons  to  mark  the  paths  they  follow. 

Many  of  the  falls  in  the  drift-covered  region  of  North 

America  are  due  to  the  turning  of  streams  from  pre-glacial 

alleys  in  such  a  way  as  to  cause  them  to  flow  over  what 

were   formerly    divides   or   rocky  spurs   between   adjacent 

streams  and  plunge  into  valleys.     In  some  instances,  also, 


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RIVERS  OF  NORTH  AMERICA 


they  have  cut  through  a  covering  of  drift  and  been  lowered 
upon  harder  rocks  beneath.  In  either  case  falls  may  result. 
Cascades  are  also  produced  where  these  streams  leave  a 
region  of  hard  rock  and  enter  areas  of  drift  which  is  more 
easily  removed. 

Niagara  Falls  came  into  existence  when  a  large  lake,  which 
formerly  flooded  both  the  Ontario  and  Erie  basins,  was 
lowered  so  as  to  be  divided  into  two  water  bodies  by  a  ridge 
trending  east  and  west,  formed  by  the  summit  of  the  Lewis- 
ton  escarpment. 

The  Falls  of  St.  Anthony  are  due  to  the  Mississippi 
having  been  turned  from  its  pre-glacial  course  by  deposits 
of  drift,  and  made  to  flew  over  the  surface  of  a  compara- 
tively thin  horizontal  sheet  of  limestone  resting  on  soft 
sandstone.  A  cataract  about  one  hundred  feet  high  was 
produced  where  the  river  left  the  q^^q.  of  the  limestone 
layer  and  plunged  into  a  pre-glacial  valley.  From  this 
escarpment  the  fall  has  receded  about  eight  miles,  leaving  a 
steep-sided  canyon,  as  in  the  case  of  Niagara.      ^ 

Shoshone  Falls,  Idaho,  wjre  produced  by  a  hard  sheet  of 
trachyte  which  Snake  River  discovered  as  it  sank  its  chan- 
nel in  nearly  horizontal  layers  of  basalt.  The  falls  have 
migrated  up  stream,  leaving  a  narrow  canyon  as  a  record  of 
the  work  already  performed. 

Many  beautiful  cascades  in  the  Rocky  and  Cascade 
Mountains,  and  others  no  less  picturesque  in  the  Sierra 
N'jvada,  are  due  to  changes  produced  by  a  former  period 
of  glaciation.  In  some  instances  in  the  Sierra  Nevada,  lari^^c 
Alpine  glaciers  flowed  down  the  main  valleys,  and  blocked 
the  streams  in  lateral  gorges  so  as  to  cause  them  to  cease 


iNEQUALITIES  IN  THE  HARDNESS  OF  ROCKS         63 


corrading.  The  glaciers  deepened  the  main  valleys,  how- 
ever, during  the  time  the  development  of  their  tributary 
streams  was  arrested,  and  when  the  ice  melted  and  water 
drainage  was  once  more  established,  the  branches  of  the 
main  streams  were  compelled  to  descend  steep  precipices 
and  in  many  instances  form  fine  cascades,  in  order  to  reach 
the  bottoms  of  the  glacier-deepened  main  valleys. 

The  ancient  glaciers,  to  which  so  many  references  have 
been  made,  brought  destruction  in  their  paths  as  they  ad- 
vanced and  left  fields  of  desolation  as  they  retreated,  but  in 
many  ways  the  beanty  of  the  region  they  occupied  was  en- 
hanced by  the  changes  they  made.  Our  greatest  debt  to 
the  vanished  glaciers,  so  far  as  the  revolutions  they  wrought 
appeal  to  o  jr  artistic  sense,  is  for  the  tens  of  thousands  of 
placid  lakes  they  left  strewn  over  the  land,  and  the  tens 
of  thousands  of  leaping  waterfalls  which  sprang  into  exist- 
ence on  their  retreat.  The  former  are  emblems  of  rest,  the 
latter  of  ceaseless  activity. 

Bluffs  Bordering  Aged  Streams. — As  previously  stated 
the  influence  of  the  unequal  yielding  of  hard  and  soft  rocks 
is  most  marked  in  the  case  of  streams  that  are  still  young, 
and  decreases  as  they  advance  in  development,  but  seldom 
entirely  disappears.  " 

Topography  being  largely  the  result  of  the  action  of 
streams,  it  follows  that  the  various  features  in  the  relief 
of  the  land  must  be  due,  to  a  marked  extent,  to  the  un- 
equal waste  of  hard  and  soft  rocks.  The  hard  rocks  stand 
as  bluffs,  ridges,  and  peaks,  while  the  soft  rocks  are  worn 
away  more  rapidly,  and  dells  and  valleys  appear.  The 
various  stages  from  youth  to  old  age,  so  characteristic  of 


\  ^  .1 


I'  ^^ 


•a 
I 


RIVERS  OF  NORTH  AMERICA 

streams,  find  a  counterpart  in  the  general  changes  in  form 
and  expression  experienced  by  the  surfaces  of  land  areas. 
A  new  land  area  may  have  a  generally  even  surface,  but  as 
time  passes,  and  topographic  maturity  is  reached,  it  becomes 
roughened,  and  if  the  elevation  has  been  great,  and  marked 
inequalities  in  tne  hardness  of  the  rocks  occur,  exceedingly 
rugged  topographic  forms  will  be  developed.  When  a  land 
area  has  been  long  exposed,  the  inequalities  of  surface  due 
to  differential  weathering  gradually  decrease,  but  except  in 
the  rare  instances  of  nearly  complete  baselevelling,  do  not 
disappear.       .         ' 

Throughout  the  courses  of  streams  that  have  passed  their 
periods  of  maturity,  and  even  after  having  developed  a 
gentle  gradient  characteristic  of  old  age,  their  valley  walls 
frequently  retain  evidences  of  the  great  topographic  diver- 
sity that  characterised  them  during  youth.  Rivers  flowing 
through  Irnds  having  in  general  all  the  characteristics  of 
topographic  old-age  may  yet  be  bordered  in  places  by  steep 
bluffs  and  overshadowed  by  towering  precipices. 

The  Highlands  of  the  Hudson,  where  the  river  valley  is 
narrow  and  bordered  on  each  side  by  rugged  mountains,  in 
contrast  with  the  wider  portions  above  and  below,  where  the 
bordering  uplands  are  less  precipitous,  »"eveal  the  influence 
of  hard  rocks  on  the  ss  "^nery  of  an  ancient  river.  The 
picturesque  "  coves  "  in  the  Southern  Appalachians,  as 
along  the  upper  course  of  the  Hiawassee,  have  been  hol- 
lowed out  in  soft  beds,  and  are  surrounded  by  piecipitous 
mountains  of  hard  rock. 

Many  bold  headlands  in  the  upper  Mississippi  valley  arc 
remnants  ot  ancient  en^inences,  rounded  and  worn  by  loiiR 


INEQUALITIES  IN  THE  HARDNESS  OF  ROCKS 


65 


exposure,  which  in  several  instances  rise  directly  from  the 
border  of  the  liver  that  sweeps  about  them  and  has  long 
since  passed  its  period  of  youth. 

,  Much  of  the  wonderfully  impressive  scenery  of  the 
Columbia  is  due  to  great  bluffs  of  basalt  which  rise  di- 
rectly from  the  river's  brink,  and  on  account  of  their 
hardness,  in  contrast  with  softer  beds  adjacent,  remained 
prominent  even  after  the  river  bad  cut  down  its  channel  to 
an  even  grade  and  become  navigable. 

The  same  sequence  of  events  was  noted  by  the  writer 
in  many  instances  wliile  ascending  the  Yukon.  That  noble 
river,  although  well  adjusted  to  the  various  rock  conditions 
it  discovered  as  it  deepened  its  channel,  and  flowing  with 
such  an  even  grade  that  it  can  be  ascended  by  steamboats 
for  over  fifteen  hundred  miles,  is  bordered  in  places,  as 
shown  in  Plate  II.,  by  magnificent  bluffs  of  hard  rock  which 
intervene  between  long  reaches  whe/e  the  valley  is  several 
miles  broad,  and  has  been  excavated  in  solter  beds. 

While  the  persistence  of  the  topographic  forms  on  the 
borders  of  river  valleys  is  conspicuous,  and  accounts  for 
many  of  the  more  prominent  features  adjacent  to  aged  rivers, 
yet  the  lives  of  many  streams  have  been  so  greatly  prolonged 
that  movements  in  the  earth's  crust  have  produced  changes 
simulating  those  just  considered.  The  rocks  crossed  by  a 
great  river  may  be  upraised  so  as  to  form  ridges,  or  even 
mountain  ranges,  athwart  its  course,  and  dam  its  waters  or 
turn  them  aside.  When  such  changes  occur,  however,  with 
sufificient  slowness  to  allow  the  river  to  deepen  its  channel 
as  fast  as  the  rocks  rise,  it  will  maintain  its  right  of  way, 
and  excavate  a  gorge  or  canyon  through  the  obstruction. 

5 


mmma 


66 


mVERS  OF  NORTH  AMERICA 


In  such  instances  a  portion  of  the  stream  will  have  the  charac- 
teristics of  youth,  while  adjacent  portions  above  and  below, 
whose  development  was  unchecked,  present  all  the  features 
of  old  age.  Rivers  which  maintain  their  right  of  way  in 
the  manner  just  cited,  and  carve  gorges  and  canyons  through 
newly  elevated  lands,  have  been  termed  antecedent  rivers  by 
Powell,  in  recognition  of  the  fact  that  they  are  antecedent 
to  the  movement  which  causes  the  rocks  to  be  elevated. 


n 


-iSt 


CHAPTER  IV 

MATERIAL   CARRIED  BY  STREAMS  IN  SUSPEN- 
,  SION  AND  IN  SOLUTION 

THE  waters  flowing  from  the  land  back  to  the  sea, 
whence  they  came  as  vapour,  carry  material  with 
them,  as  is  well  known,  in  two  distinct  ways:  namely,  in 
suspension  and  in  solution.  The  debris  carried  in  suspen- 
sion and  rolled  along  the  bottom,  or  the  visible  load,  as  it 
may  be  termed,  sooner  or  later  finds  its  way  to  the  sea  and 
forms  stratified  deposits.  The  material  dissolved  by  the 
waters  during  their  excursion  through  the  air  and  over  the 
land,  or  their  invisible  load,  goes  to  increase  the  salinity  of 
the  sea  and  to  supply  marine  plants  and  animals  with  sub- 
stances necessary  for  their  growth. 

THE   VISIBLE   LOADS   OF  STREAMS 

The  material  transported  mechanically  by  streams  may  be 
divided  into  two  classes:  ist,  the  portion  rolled  and  pushed 
along  the  bottom;  and,  2d,  the  portion  lifted  well  above 
the  bottom  and  carried  forward  in  suspension.  The  divid- 
ing plane  between  these  two  classes  is  indefinite,  as  much 
of  the  material  moved  along  the  bottom  makes  short  up- 
ward excursions,  and  the  fine  particles  normally  carried 
forward  in  suspension  from  time  to  time  rest  on  the  bottom. 

67 


\     '■ 


V'*^   'T- 


■  f 


68 


RIVERS  OF  NORTH  AMERICA 


■'.m 


fi!  1 

1 1 


Bottom  Load. — Concerning  the  manner  in  which  the  bot- 
tom load,  as  it  may  be  termed,  is  moved,  and  the  amount  of 
such  transportation  in  a  given  stream,  but  little  information 
is  available. 

If  we  watch  a  clear  stream  supplied  with  sand  or  gravel  of 
such  size  that  the  current  has  power  to  move  it,  it  will  be 
seen  that  the  debris  does  not  advance  as  a  continuous  sheet, 
but  rather  as  a  succession  of  wave-like  forms.  The  action 
of  the  water-current  in  this  respect  is  similar  to  the  be- 
haviour of  air-currents  when  moving  over  c'-'v  sand.  The 
ripple-like  ridges  on  the  bottom  of  streams  are  frequently 
and  probably  always  broad  in  reference  to  their  height. 
The  up-stream  slope  of  each  ridge  is  gentle  and  its  down- 
stream border  short  and  precipitous.  Grains  of  sand  are 
moved  over  the  broad  gently  ascending  surface  and  rolled 
down  its  steep  down-stream  margin.  At  the  base  of  the 
steep  border  of  each  ripple-like  sheet,  there  are  secondary 
currents  caused  by  the  plunging  of  the  water,  and  the 
particles  forming  the  bottom  are  there  disturbed  and  c  rried 
onward  and  the  process  repeated.  When  the  material  at 
the  bottom  is  in  excess  of  the  transporting  power  of  the 
stream,  sedimentation  takes  place,  and  cross-stratified  or 
current-bedded  accumulations  result;  but  if  the  bottom  cur- 
rent is  under-loaded,  the  material  is  carried  forward  by  being 
removed  from  the  up-stream  margin  of  a  broad  ripple-like 
sheet,  and  re-deposited  on  its  steep  down-stream  margin. 

The  process  just  described  goes  on  at  the  bottom  of  clear 
streams,  and  illustrates  the  fact  that  such  streams,  contrary 
to  what  is  sometimes  stated,  have  power  to  cort-ade.  It  is 
only  when  their  bottoms  are  swept  clean  of  all  grains  of  such 


MATERIAL    CARRIED  BY  STREAMS 


69 


sfze  as  are  within  the  capacity  of  the  stream  to  sweep  away, 
that  mechanical  corrasion  ceases. 

These  statements  concerning  the  bottom  loads  of  streams 
may  be  said  to  be  qualitative,  inasmuch  as  measures  of  the 
amount  of  material  thus  transported  are  lacking.  It  is  diffi- 
cult and  at  present  seemingly  impossible  to  ascertain  how 
much  material  a  large  river  is  moving  in  the  manner  just 
considered.  Bottom  transportation  will  evidently  vary  with 
changes  in  conditions,  being  favoured  by  swiftness  of  cur- 
rent and  the  character  of  the  debris  available  for  transporta- 
tion. Variations  probably  also  occur  in  reference  to  the 
amount  of  material  a  stream  carries  in  suspension.  If  a 
stream  is  heavily  charged  with  silt,  the  friction  of  flow  will 
be  increased,  and  as  this  friction  is  greatest  in  proportion  to 
rate  of  flow,  near  the  bottom,  it  is  to  be  expected  that  bot- 
tom transportation  will  be  checked  while  transportation  in 
suspension  is  still  actively  progressing.  It  would  seem, 
therefore,  as  if  bottom  transportation  is  favoured  by  de- 
crease of  material  in  suspension ;  or,  other  conditions  being 
the  same,  clear  streams  have  a  greater  power  to  move  bot- 
tom loads  than  muddy  streams.  I  must  confess,  however, 
that  this  is  theory  rather  than  a  deduction  from  observations 
and  experiments,  and  the  reader  is  invited  to  test  the  con- 
clusion for  himself. 

It  is  evident  that  the  principal  conditions  favouring  bot- 
tom transportation  are  velocity  and  volume  of  water.  As 
velocity  increases  with  declivity,  we  should  expect  that 
high-grade  streams  would  move  proportionately  heavier 
loads  along  their  bottoms  than  low-grade  streams,  other 
conditions  being  the  same.     The  ratio  of  bottom  load  to 


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/f"-"** 
-r'-  '$ 


m 


*' 


r 

-4- 


^o 


RIVERS  OF  NORTH  AMERICA 


total  transportation  should  be  greater  during  floods  than 
during  lowwater  stages.  Observation  seems  to  confirm 
these  conclusions.  ^ 

The  proportion  of  bottom  load  to  the  amount  of  material 
carried  by  a  stream  in  suspension  is  dependent  largely  on 
the  character  of  the  debris  within  the  reach  of  the  stream — 
that  is,  whether  it  is  fine  or  coarse ;  but  in  general  it  seems 
true,  as  just  stated,  that  the  bottom  load  in  swift  streams  is 
greater  in  proportion  to  the  amount  of  material  in  suspen- 
sion, than  is  the  case  in  slower  streams  under  similar  second- 
ary conditions. 

Although  the  manner  in  which  bottom  loads  are  carried 
is  not  thoroughly  understood,  and  the  amount  of  such  trans- 
portation difficult  to  determine,  the  fact  remains  that  much 
of  the  energy  of  streams  is  consumed  in  rolling  debris  along 
their  bottoms.  In  many  measures  of  the  rate  at  which 
streams  are  removing  rock  debris  from  their  drainage  basins, 
the  quantity  moved  along  their  bottoms  is  not  considered. 
For  this  reason  most  estimates  of  the  rate  at  which  land 
areas  are  being  lowered  by  denudation  require  important 
modifications. 

Measures  of  Material  in  Suspension. — The  methods  em- 
ployed for  ascertaining  the  amount  of  sediment  carried  by 
a  stream  in  suspension  are  illustrated  by  the  careful  work 
done  in  this  connection  by  Professor  Forshey,  during  the 
survey  ot  the  Mississippi  by  the  United  States  Topographic 
Engineer  Corps.'  Stations  were  selected  near  CarroUton,  a 
short  distance  above  New  Orleans,  one  about  three  hundred 


'  Humphreys  and  Abbot,  Report  upon  the  Physics  and  Hydraulics  of  the 
Mississippi  River,  p.  137,  1861. 


MATERIAL   CARRIED  BY  STREAMS 


71 


feet  from  the  east  bank  of  the  river,  the  next  in  mid-stream, 
and  a  third  about  four  hundred  feet  from  the  west  bank. 
The  high-water  depths  at  these  stations  were  100,  100,  and 
40  feet  respectively.  Samples  of  water  were  collected  daily 
at  surface,  mid-depth,  and  bottom  at  the  first  two  stations; 
and  at  surface  and  bottom  at  the  third  station  for  a  period 
of  one  year.  During  the  succeeding  year,  the  ratio  between 
the  sediment  contained  in  the  water  at  any  one  station  and 
that  contained  in  the  entire  cross-section  of  the  river  having 
been  ascertained,  one  sample  was  taken  each  day  from  the 
surface  at  the  station  near  the  east  bank. 

The  samples  from  below  the  surface  were  secured  by 
means  of  a  small  keg  heavily  weighted  at  the  bottom  and 
provided  at  each  of  its  ends  with  a  large  valve  opening  up- 
ward. These  valves  allowed  a  free  passage  to  the  water 
while  the  keg  was  sinking  to  the  required  depth,  but  pre- 
vented its  escape  while  being  drawn  up.  When  the  keg 
reached  the  surface,  the  water  contained  in  it  was  thor- 
oughly stirred  and  a  bottle  filled  from  it.  The  sediment 
contained  in  the  water  samples  was  subsequently  filtered 
out  and  weighed  after  drying. 

From  the  tabulated  results  of  the  first  year's  work  re- 
ferred to,  it  was  found  that  the  greatest  amount  of  sediment 
was  carried  in  June,  during  the  annual  high-water  stage  of 
tne  river,  the  weight  of  sediment  then  being  -^^  of  the 
weight  of  the  river  water  containing  it;  the  minimum  was 
obtained  during  the  low-water  stage  late  in  October,  the 
ratio  of  weight  of  sediment  to  weight  of  wa*:er  then  being  as 
I  to  6383.  The  mean  for  the  year  was  ^-gV?,  or  one  ton  of 
sediment  to  1808  tons  of  water. 


*■ 


72 


RIVERS  OF  NORTH  AMERICA 


A  discussion  of  a  still  larger  number  of  observations, 
made  under  the  direction  of  Humphreys  and  Abbot,  gave 
the  ratio  of  i  of  sediment  to  1500  of  water  by  weight,  and 
of  about  I  to  2900  by  volume. 

The  variation  in  the  amount  of  sediment  with  positions  in 
the  stream  is  indicated  in  the  following  table  of  the  weekly 
means  for  the  two  months  of  highest  and  lowest  water 
respectively:  , 

SEDIMENT  IN  THE  MISSISSIPPI  AT  CARROLLTON 


FIRST   POSITION. 

SECOND    POSITION. 

THIRD    POSITION. 

NUMBER  OF  WEEK. 

u 

a 

«  0. 

0.407 
0.507 

0.960 

0.570 

d 
0 

0 
« 

0.187 
0.510 
0.940 
0.557 

0.548 

8 

u 

. 

B 
0 

0 
n 

u 

3 

"3  a. 

First     in  June,  1851 

Second"      "       "    

Third     "      "       "    

Fourth  "      "       '•    

0.345 
0.456 
0.917 
0.498 

0.365 
0.477 
0.731 
0.528 

0.415 

0.515 
0.98 1 

0.597 

0.410 

0.517 
1. 105 
0.601 

0.285 
0.365 
0.666 
0.427 

0.390 

0.457 

1.046 

0.536 

0.365 
0.442 
0.447 
0.452 

Mean  for  June 

0.559 

0.6II 

0.525 

0.627 

0.658 

0.436 

0.607 

0.426 

First     in  October,  1851.. 

Second 

Third    " 

Fourth' 

0.137 
0.120 
O.IOO 
0.068 

0.187 
0.169 
0.132 
0.096 

0.220 
0.170 
0.136 
0.106 

0.125 
0.109 
0.097 
0.059 

0.096 

0.215 

0.193 

0.146 
0.II5 

0.235 
0.220 
0.159 
o.ii6 

0.096 
0.107 
0.089 
0.061 

0.265 

0.235 

0.195 
0.136 

0.170 
0.092 
0.071 
0.081 

Mean  for  October 

0.106 

0.146 

0.158 

0.172 

0.182 

0.088 

0.208 

0.104 

The  figures  denote  the  number  of  grammes  of  dry  sediment  contained  in  600 
grammes  of  river  water. 

Knowing  the  amount  of  water  discharged  annually  by  the 
Mississippi  and  the  proportion  of  sediment  contained  in  it, 
the  amount  of  material  carried  by  the  stream  each  year  in 


MATERIAL    CARRIED  BY  STREAMS 


73 


suspension  niay  be  readily  computed.  The  mean  annual 
discharge,  as  determined  by  the  survey  in  charge  of  Hum- 
phreys and  Abbot,'  is  19, 5 oc,cxx), 000,000  cubic  feet,  and  the 
amount  of  solid  matter  carried  in  suspension  812,500,000,- 
000  pounds."  The  average  specific  gravity  of  this  material 
is  about  1.9 ;  with  this  density,  the  sediment  carried  annually 
would  occupy  6,718,694,  400  cubic  feet,  or  suflficient  to  cover 
one  square  mile  to  the  depth  of  241  feet. 

In  addition  to  the  silt  carried  in  suspension,  it  has  been 
estimated  by  the  engineers  cited  above,  that  the  amount  of 
sand  and  gravel  rolled  along  the  bottom  and  contributed 
each  year  to  the  filling  of  the  Gulf  of  Mexico  is  about 
750,000,000  cubic  feet ;  making  the  total  visible  load  carried 
by  the  river  each  year  about  7,468,694,400  cubic  feet,  or 
sufficient  to  cover  one  squar-^  mile  to  a  depth  of  268  feet. 

The  Mississippi  has  been  more  carefully  studied  than  any 
other  river  in  North  America,  but  it  is  well  known  that 
other  streams  are  doing  a  similar  work.  In  many  streams 
the  proportion  of  material  in  suspension  to  the  amount  of 
water  is  greater  than  in  the  lower  Mississippi ;  while  rivers 
might  be  selected,  as,  for  example,  the  St.  Lawrence,  in 
which  the  percentage  of  sediment  is  much  less.     An  iiispec- 

'  Report  on  the  Mississippi  River ^  p.  149.  If  I  understand  this  portion  of 
Humphreys  and  Abbot's  report  correctly,  the  above  measures  do  not  include 
the  three  outlet  bayous,  which  leave  the  main  river  above  New  Orleans.  It  is 
stated  on  page  93  of  the  report,  that,  including  these  bayous,  the  annual 
discharge  is  2i,300,ooo,cxx),ooo  cubic  feet  of  water. 

'^Taking  the  specific  gravity  of  water  as  i,  the  relative  weight  of  coarse 
river-sand  is  1.88;  fine  sand,  1.52  ;  clay,  1.90;  alluvial  matter,  from  1.92  to 
2.72.  A  cubic  foot  of  water  weighs  62.5  lbs.  ;  of  coarse  sand,  117. 5  lbs.  ;  fine 
sand,  95  lbs.  ;  clay,  118.75  lbs.  ;  alluvial  matter,  120  to  170  lbs.  ;  silt,  103 
lbs.  W.  H.  Wheeler,  Tidal  Rivers,  p.  62.  Published  by  Longmans,  Green, 
&Co.,  1893.  ,_„^,,__,    .  , 


74 


RIVJ'lKS   OF  A'OA'77/  AMERICA 


tion  of  the  following  tabic,  compiled  by  C.  C.  Babb,'  in 
which  data  concerning  the  amount  of  material  that  is  being 
carried  hy  several  large  rivers  are  presented,  shows  that  the 
Mississippi,  although  commonly  recognised  as  a  muddy 
stream,  holds  a  smaller  percentage  of  silt  in  suspension  than 
several  other  rivers  with  which  it  may  be  compared. 

disciiar(;e  and  skdimknt  ok  lakoe  rivkrs 


2 

ii.'>43 
1,244  OCX) 
3o,(xxj 
i5o,(xx) 
34,8cx) 
a7,i(x) 

320, 3(K) 

I,I(X),CX)0 

I35,(XX> 

'-•  i.     ? 
•/,  ■«  * 

M  U 

n 

20,160 

6u»,(xx) 

i,7(x> 

I50,(xx) 

65,850 

62,2<X) 

315, 2(X) 

ii3,ix)o 
475.000 

■      HRIIIMiiNT. 

RIVER. 

1 

h 

r 

S.SS-'-aso 

4o6,25o,(xx) 

3,83o,cyx) 

14.782,500 

3^,<XX),OCMl 

67, tXMI, ()(><) 

108, (MX), (XX' 

54,(HK),()lXy 

2(;l,430,(XX) 

06  B 

«:   3.575 
i:   i.S'x^ 
1 :       2(>l 
1 :  10, (XX) 

1:    «,775 
I :       ()fx) 
i:   2.880 
l:  2,050 
i:   1,610 

Height  of  col- 
umn. 00:   sq. 
mile  base. 
Feet. 

0-3 

Potomuc 

Mississippi 

Rio  (irande 

Uruguay 

Rhone 

4.0 

341-4 

2.8 

10.6 

.fx)433 

.(K)22;, 
.(KJII'j 
.0«X)85 
.0I07R 

Po 

5().o     1  .oii3() 

93.2     i  .0354 

3^.8        .(XK)42 

2(x;,o     1  .02005 

Danube 

Ni)e 

Irrawaddy 

Mean 

334.693 

301,468 

io<).640,072 

i:  2,731 

76.65 

.oof)i4 

Estimates  .similar  to  those  jjiven  in  the  above  table  have 
been  published  by  .several  geologists.  One  series  of  these, 
probably  as  reliable  as  any,  by  Archibald  Geikie,'  is  here 
copied  for  the  purpose  in  part  of  showing  that  tlie  observa- 
tions now  available  concerning  the  work  of  streams  are  de- 
fective. Even  the  areas  of  liydrographic  basins  are  stated 
differently    by  different    writers,   and    with  perhap3  a  few 


'  Srifficf,  vol.  xxi.,  p.  343,  June,  JS()3. 

*  Tixt-Book  0/  Gtotogy,  ad  .ditiun,  p.  438.     .MacmilUn  &  Co.,  1885. 


MATERIAL   CAKHfED  jIV  STREAMS 


75 


exceptions  the  n.casurcs  ^'ivcn  of  the  annual  discharge  of 
large  river:.,  the  amount  of  sediment  they  carry,  etc.,  should 
be  considered  as  subject  to  corrections. 

SKDIMKNT  OF  RlVrUS 


RIVKR. 


Mississi|)ni  ... 
(ian(;cH  (Upper) 
Iloaiig  llo  . .  . . . 

Klionc , 

Danube 

Vo 


ANKA    OK    IIAKIN    IN 
N^UAKIt    MII.RH. 


1,147.000 
143, (KX3 
7<)<),<XX) 

'J<j,(MM) 
2.'l4,0«H) 

3<>,(KK) 


ANNIMI    HmCIIAHriK 

tir   HKDIMKNI    IN 

CUIIIC    rKKT, 


7,4S(}.2r.7,2(X) 
^»,3'>«.'>77.440 

I7,S20,0<«),OCfj(?) 

fHK),38i,8(K) 
',5«'>.I37.«X) 


raAcn-ioN  or  riiT 

(IF   KOI  K  nv  WHICH 

TIliC  ARKA   IIHAINRD 

IN    l.dWKNKIi    IN 

ONR    VKAM. 


m'lii 


A  brief  discussion  of  the  rate  at  which  land  areas  are  being 
lowered  by  the  removal  of  material  by  streams  will  be  given 
after  the  measures  of  mineral  matter  in  solution  have  been 
considered.  • 

THE   INVISIBLK    LOADS   OK   STREAMS 

Water  as  ii  reaches  the  land  as  rain,  snow,  dew,  etc.,  is 
never  chemically  pure,  but  contains  both  organic  and  in- 
organic matter  in  solution  and  dust  particles  in  suspension. 
The  substances  most  commonly  occurring  in  solution  in 
rain-water  are  shown  by  the  following  analysis  of  a  .sample 
collected  near  London,  Kngland  ' : 

Or|;anic  carbon 99  part  in  i,0(k),uou  of  water. 

Organic  nit ro|ren .22       "  "  " 

Ammonia 50       '*  *'  " 

Nitrogen  as  nitrates  and  nitrites. .         .07       "  "  "' 

Chlorine 6,30  parts  in         "  " 

Total  solids »^-M-*^  3«i.5o       "  '*  "         ^~ 

'(quoted  by  W.  P.  Mason,  IVatfr  Supf>ly,  p.  304.     John  W'ley  &  Sons,  1896. 


m 


» 

It 
t 

I 


I 


■^ 
^ 


1 
■I 


'4 1:: 

•3  111 


76 


RIVERS  OF  NORTH  AMERICA 


Examinations  for  chlorine  in  water  samples  representing 
the  average  condition  of  the  rain-water  at  Troy,  New  York, 
for  one  year,  gave  a  mean  of  1.64  parts  in  a  million.  That 
is,  each  million  pounds  of  rain-water  contained  1.64  pounds 
of  chlorine  in  solution.'  , 

The  impi'.rities  in  rain-wat^r  vary  in  character  and  amount 
in  different  localities.  In  general  they  are  greatest  near 
cities,  and  least  in  the  open  country  at  a  distance  from  vol- 
canoes, gas  springs,  etc.  They  also  vary  with  climatic  con- 
ditions, being  greatest  in  arid  and  least  in  humid  regions, 
and  greater  in  dry  than  in  wet  seasons.  The  amount  of 
common  salt  is  large  near  the  sea,  and  normally  decreases 
inland,  but  probably  reaches  a  maximum  in  the  neighbour- 
hood of  saline  lakes  and  over  salt  deserts.  Rain-water, 
then,  comes  to  the  earth  with  its  solvent  power  increased 
by  the  presence  of  various  substances  washed  out  of  the  air, 
but  its  ability  to  take  up  mineral  matter  in  solution  is  greatly 
increased  as  it  flows  over  the  land. 

The  soil  usually  contains  organic  matter  which  is  easily 
dissolved.  The  most  common  substances  thus  added  to  the 
water,  which  enhance  its  chem.ical  activity,  are  carbonic  acid 
or  carbon  dioxide  (CO3),  and  a  large  group  of  organic  acids, 
known  as  the  humus  acids;  these,  however,  are  unstable, 
and  soon  change  to  carbon  dioxide.  The  organic  acids  are 
derived  mainly  from  the  decay  of  vegetation,  but  in  part 
are  of  animal  origin.' 

The  percentage  of  the  organic  acids  taken  in  solution  by 

'  VV.  P.  Mason,  Water  Supply,  p.  205. 

*  A.  A.  Julien,  "  On  the  Geological  Action  of  the  Humus  Acids."  in  Amn- 
ican  Association  for  tkt  Advamtmtnt  0/  Sdena,  Proceedings,  vol.  xxviii.,  pp. 
3i»-4lo,  1879. 


MATERIAL   CARRIED  BY  STREAMS  .    J^ 

a  given  quantity  of  water  percolating  through  the  soil  varies 
with  different  localities,  being  greatest  when  decaying 
vegetation  is  most  abundant  and  where  the  temperature  is 
high.  In  all  portions  of  the  earth's  surface,  however,  the 
water,  on  coming  in  contact  with  the  soil  or  with  solid  rocks, 
has  the  power  to  dissolve  portions  of  them.  The  water 
which  runs  over  the  surface  and  is  gathered  quickly  into 
streams  has  less  opportunity  to  take  up  mineral  matter  in 
solution  than  that  which  percolates  through  the  soil  and  in 
many  instances  descends  into  the  hard  rocks  beneath  and 
comes  to  the  surface  again  as  springs.  The  water  flowing 
quickly  over  the  surface  and  that  following  more  or  less  ex- 
tensive underground  courses  are  commingled  in  the  streams, 
and  send  their  combined  tribute  of  dissolved  matter  to  the 
sea.  Chemical  denudation  thus  assists  the  mechanical  ac- 
tion of  flowing  water  in  lowering  the  land,  and  is  an  import- 
ant factor  in  the  process. 

The  rate  at  which  rocks  are  dissolved  varies  not  only  with 
th^  rain-fall,  with  the  amount  of  organic  acids  in  surface  and 
subterranean  waier,  and  with  temperature,  but  is  influenced 
especially  by  the  nature  of  the  rocks  in  variou«  regions.  The 
solution  of  mineral  matter  in  general  is  greater,  othei  con- 
ditions  remaining  the  same  the  higher  the  temperature, 
although  this  does  not  apply  to  limestone,  and  is  greatest 
where  the  rocks  are  composed  of  easily  soluble  minerals. 
For  these  reasons  the  chemical  composition  of  river-watc 
varies,  but  the  departure  from  a  mean,  as  shown  by  a  la'-ge 
number  of  analyses,  is  less  than  might  at  first  be  expected. 

By  the  time  the  surface  waters  have  united  to  form  rilU 
they  contain  suflficient  mineral  and  organic  matter  to  give 


■•a* 


lll.ll 


78 


RIVERS  OF  NORTH  AMERICA 


them  a  complex  chemical  composition.  Throughout  their 
journeys  to  the  ocean,  as  they  form  brooks,  creeks  and 
rivers,  and  especially  when  travelling  underground,  they  be- 
come more  and  more  highly  charged  with  dissolved  mineral 
matter.  The  longer  the  waters  are  in  contact  with  soil  and 
rocks,  and  with  the  finely  divided  material  held  by  them  in 
suspension,  temperature  conditions,  etc.,  remaining  the 
same,  the  more  highly  charged  they  become  with  substances 
in  solution.  Evaporation  also  tends  to  concentration,  but 
this  process,  particularly  in  humid  regions,  is  more  or  less 
completely  counteracted  by  direct  precipitation. 

River-waters,  filtered  of  all  material  in  suspension,  and 
evaporated  to  dryness,  leave  a  solid  residue,  which  is  the 
principal  portion  (the  more  volatile  substances  escaping)  of 
the  foreign  matter  previously  held  in  solution.  These 
waters  are  fresh  in  the  every-day  use  of  the  term,  but  in 
fact  owe  their  agreeable  taste  and,  to  a  certain  extent,  their 
health-giving  qualities,  to  the  mineral  salts  and  gases  con- 
tained in  them.  In  Table  A,  analyses  are  given  of  the 
waters  of  a  number  of  American  rivers,  which  show  that  the 
principal  substances  in  solution  are  calcium  and  carbonic 
acid,  probably  combined  as  calcium  bicarbonate.  In  some 
instances,  however,  as  in  the  case  of  Jordan  River,  Utah, 
calcium  sulphate  is  in  excess  of  all  other  salts. 

From  a  large  number  of  analyses  of  water  samples  ob- 
tained from  the  rivers  of  Canada  and  the  United  States,  it 
has  been  found  that  the  average  amount  of  total  solids  in 
solution  is  o.  1 5044  part  in  a  thousand  by  weight ;  of  this 
material,  0.056416  part  in  a  thousand  is  calcium  carbonate. 
In  a  table  of  forty-eight  analyses  of  European  river-waters 


l-i-j 


V- 


^ai-di^nA  aoj 


H 


I... 


,flO«llllH  '■ «»l«««i9t '. 


itnblvMlni 


.8^61  ;8  ,!K|9*  \  iat\K  .1^81  ,>.«  jaO  I 


.    5    lov 


.<iu*\  .III A     .i 


f  .3 ; snuW  ,H  '  .  .nojqwJ  .T 


ii-vro 


:       M^l.l-1 

1    »r>'»o- 

«£♦«» 

11  uw. 

J!J?«0 

'     ?.T'™'> 

..     I^««l>» 

.    *£?«». 

"j.     t*MO. 

.A.  .< 


■1<!VS 


l... 


•  i^t  t  •;#  •!  •«»»*«««*^*«**#*f  % 


4|'M 


.•rtvfl 


.mmT 


}j  1t«^  ]| 


.•ra    H  tt»>«^ 


aft.^1 


'»rr 


TABLE  A.— ANALYSES  OF  AMERICAN   R 
[Reduced  to  parts  per  looo  by  Dr.  H.  J.  Voi 


Sodium,  Na 

Potassium,  K 

Calcium,  Ca, 

Magnesium,  Mg 

Chlorine,  CI 

Carbonic  acid,  COa 

Sulphuric  acid,  SO4 

Phosphuric  acid,  HPO, 

Nitric  acid,  NO, 

Silica,  SiO, 

Alumina,  A1|0| 

Sesquioxide  of  iron,  Fe,Oa. . 

Sesquioxides  of  iron  and  alfmina,  Fe,0»  and  A1,0| 

Ses(|uioxidcs  of  iron  and  mangancM,  Fe|0,  and  MnaO, 

Carbonates  of  iron  and  manganese,  FeCO,  and  MnCO, 

Oxide  of  iron,  F<!0 

Oxide  of  manganese,  MnO 

Hydrogen  in  bicarbonates,  H 

Chloride  and  sulphate  of  sodium,  NaCI  and  NaSO« 

Am.-nonia,  NH4 

Organic  matter 

Carbonates  and  sulphates  of  Na,  K,  and  Mg 


pwp* 


'SES  OF  AMERICAN   RIVER-WATERS. 
ts  per  1000  by  Dr.  H.  J.  Von  Hoesen.] 


ee,  Ohio. 

MbsUsippi  — 

Hydrant ;  city 

water -works. 

New  Orleans, 

La. 

Otttwu 

St.  Ann's  Lock, 
Montreal,Can. 

Mar.  9,  1854  . . . 

T.  S.  Hunt.... 

Geol.  0/  Can- 
ada, 1863,  p. 
567. 

Passaic 

4    miles   above 
Newark,    N. 

Rio      Grande, 
del  Norte. 

Fort         Craig, 
New  Mexico. 

Sacramento .  . . 

Hydrant,   Sac- 
ramento, Cal. 

Sept.,  1878 

W.J.Jones.... 
Xept.  Cal.  State 

Board  Health, 

1878. 

St.  Lawrence. 

S.    side    Point 
d(>  Cascades. 

Mar.  30, 1863  •  • 

T.  S.  Hunt.... 

Geol.  0/  Can- 
ada, 1863,  p. 
567, 

Humboldt 

3attle  Mt., 
Nev. 

Dec,  187a 

T.  M.  Chatard. 

U.     S.     Geol. 
Surv.  Mono- 
graph, xi.,  p. 
41. 

Truckee 

1  Lake     Tahoe, 
Nev. 

Oct.,  1872 

F.W.  Clarke.. 

U.      S.     Geol. 
Surv.  Mono- 
graph, xi., p. 

4a. 

Walker 

Mason  VaUey, 

Nev. 

Oct.,  i8ja. 

F.  W.  Clarke. . 

U.     S.     Geol. 
Sum.  Mono- 
graph, xi.,  p. 

40. 

— 

Mohawk 

Urica,  N.Y.... 

Genesee. 

Utah  L» 

Nov.,  187 

F.  W.  CI 

Bulletin 
U.   S. 
Surv., 

ke.... 
3 

Rochester, 

N.  Y. 

W.J.  Jones... 

Re(t.  La.    .St. 
Board    of 
Health,  i88a, 
p.  370. 

E,  N.  Horsford. 

Geol.  0/  N.  J., 

1868,  p.  703. 

O.Loew 

U.     S.     Geog. 
Survey  West 
0/  lor  'h  M., 
vol.    IH.,    p. 
576. 

Chandler. 
a/  Toledo 
r-W^orks, 

■irlce.. 
W.  9, 

p.  29, 
.0178 

C.  F.  Chandler 
JohnsoH's    Cy- 

clopedia,\o\. 

iv. 

C.  F.  Chandler. 

Johnson's  Cy- 
clopedia, vol. 
iv. 

.00161 
■00309 

.0310 

.00239 
.ooijn 
.0099a 
.00161 
.00076 

•<w»55 
.00194 
Trace. 

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Trace. 

Trace. 

Trace. 

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.00585 

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loo 

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1 

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MATERIAL   CARRIED  BY  STREAMS 


*Sv.:' 


79 


given  by  Bischof,'  the  average  of  total  solids  in  solution  is 
0.2127,  and  the  average  of  calcium  carbonate  o.  1139  part 
per  thousand.  From  the  analyses  of  thirty-six  European 
river-waters,  published  by  Roth,"  including  some  of  those 
tabulated  by  Bischof,  the  average  of  total  solids  is  0.2033, 
and  of  calcium  carbonate  0.09598  part  per  thousand.       >' 

In  both  American  and  European  river-waters,  so  far  as 
can  be  determined  from  the  data  in  ha.d,  the  average  of 
total  solids  is  0.1888,  and  of  calcium  carbonate  0.088765 
part  per  thousand.  These  figures  may  be  assumed  to  repre- 
sent the  average  of  the  solids  in  solution  in  the  waters  of 
normal  rivers.  It  will  be  noticed  that  the  average  for  cal- 
cium carbonate  is  nearly  one-half  the  average  for  total  solids. 

Knowing  the  annual  discharge  of  a  river  and  the  percent- 
age of  mineral  matter  carried  in  solution,  we  can  ascertain 
the  amount  of  dissolved  matter  that  the  river  contributes 
annually  to  the  ocean,  or  enclosed  lake  into  which  it  flows. 
To  one  unfamiliar  with  studies  of  this  nature,  the  amount 
of  rock-forming  material  thus  annually  transported  by  a 
large  river  in  an  invisible  state  is  astonishing.  The  follow- 
ing table,  showing  the  total  amounts  of  solids  in  solution 
carried  by  certain  rivers,  has  been  compiled  from  various 


5,816,805  tons  per  year. 
8,290,464 


sources:  • .     • 

Rhine 

Rhone  

Danube 22,521,434 

Thames 613,930 

Nile 16,950,000 

Croton 66,795 

Hudson 438,000 

Mississippi 112,832,171 

'  Chemical  Geolof^y,  vol.  i.,  pp.  76,  77.  English  edition,  London,  1854. 
^  AUgemein  und  chemiiche  Gtologie,  vol.  i.,  pp.  456,  457.     Berlin,  1879. 


\m 


19^^^. 

r»  *' 


'%     > 


\ 

8o 


HiVEHS  OF  NORTH  AMERICA 


The  fact  that  streams  transport  great  quantities  of  dissolved 
mineral  matter,  derived  from  the  rocks  in  the  ba='ns  they 
drain,  may  be  shown  by  computin?^  the  numbers  of  tons  of 
material  in  solution  in  a  cubic  mile  of  river-water.  This 
has  been  done  by  John  Murray,'  and  the  result,  based  on 
the  average  composition  of  the  waters  of  nineteen  of  the 
principal  rivers  of  the  world,  is  given  below: 

MATERIAL  IN  SOLUTION  IN  ONE  CUBIC  MILE  OF  AVERAGE 

RIVER.  WATER* 

CONSTITUENTS.  TONS  IN  CUBIC  MILE, 

Calcium  carbonate  (CaCOj,) 326,710 

Magnesium  carbonate  (MgCOg) 112,870 

Calcium  phosphate  (CagPgOg) 2,913 

Calcium  sulphate  (CaS04) 34.301 

Sodium  sulphate  (NagS04) 31,805 

Potassium  sulphate  (KaSOi) 20,358 

Sodium  nitrate  (NaNOg) 26,800      ; 

Sodium  chloride  (NaCl) 16,657 

Lithium  chloride  (LiCl) 2,462 

Ammonium  chloride  (NH4CI) 1,030 

Silica  (SiOa) 74,577 

Ferric  oxide  (FejOg) , 13,006 

Alumina  (AlgOg) I4,3i5 

-           Manganese  oxide  (MngOj) 5,703 

Organic  matter 79,020 

Total  dissolved  matter 762,587 

It  has  also  been  computed  by  Murray,  and  published  in 
the  article  just  cited,  that  the  volume  of  water  flowing  to 
the  sea  in  one  year,  including  all  the  land  areas  of  the  earth, 
is  about  6524  cubic  miles.  From  the  average  chemical  com- 
position of  river-water,  it  follows  that  about  4,975,117,588 
tons  of  mineral  matter  in  solution  are  being  removed  annu- 

'  Scottish  Geographical  Magazine,  vol.  iii.,  p.  76,  1887. 

*  Acids  and  bases  combined  according  to  the  principles  indicated  by  Bunscn. 


■Sil 


MATERIAL    CARRIED  BY  STREAMS 


8i 


ally  from  the  land  area  of  the  earth.  This  process  of 
removing  material  of  the  land  in  solution  has,  very  properly, 
been  termed  chemical  denudation. 

It  is  instructive  to  follow  the  history  of  the  material 
carried  in  solution  by  rivers,  and  to  see  what  changes  occur, 
especially  in  inland  seas  where  ordinary  river-waters  are 
concentrated  by  evaporation,  and  in  many  instances  the  salts 
they  contain  precipitated  in  a  crystalline  form,  and  to  ex- 
tend such  studies  to  the  ocean.  Another  fruitful  line  of 
investigation  in  this  connection  is  the  manner  in  which 
mineral  matter  in  solution  is  eliminated.  This  occurs  in 
part,  as  just  mentioned,  by  chemical  precipitation,  but  is 
effected  to  an  equally  great  extent,  and  from  dilute  solu- 
tions, through  the  action  of  plant  and  animal  life.  These 
interesting  studies,  however,  lie  beyond  the  scope  of  our 
present  thesis. 


RATE   OF  LAND   DEGRADATION 


m 
to 
h, 
n- 


Measures  of  the  amount  of  material  carried  by  streams 
both  mechanically  and  in  solution  furnish  a  means  of  ap- 
proximately determining  the  rate  at  which  the  surface  of 
the  land  is  being  degraded. 

Mecha7tical  Degradation. — As  shown  in  the  table  on  page 
74,  the  amount  of  silt  carried  annually  by  the  Mississippi,  if 
taken  uniformly  from  the  area  it  drains,  would  lower  it  ^-jVff 
of  a  foot.  That  is,  considering  only  the  material  carried  in 
suspension,  the  basin  is  now  being  lowered  at  the  rate  of 
one  foot  in  5376  years.  If  we  take  into  account  also  the 
material  rolled  along  the  bottom,  computed  to  be  750,000,- 


» 

m- 
•' 
»  ■ 

I-' 

i 

I  ^' 


1 


mssmm 


mmm 


% 


82 


RIVERS  OF  NORTH  AMERICA 


ooo  cubic  feet  per  year,'  we  find  that  the  basin  is  being 
Jowered  at  the  rate  of  one  foot  in  4638  years. 

Studies  of  the  Potomac  River,  conducted  by  the  United 
States  Geological  Survey,  have  determined  the  fact  con- 
cerning that  river  presented  in  the  table  on  page  74.  As- 
suming that  one  cubic  foot  of  the  silt  carried  by  the  Potomac 
weighs  100  pounds,  the  average  annual  air>ount  transported 
would  cover  one  square  mile  to  a  depth  of  3.98  feet.  If 
this  amount  should  be  taken  uniformly  from  all  parts  of  the 
area  drained,  it  would  be  lowered  0.0043  of  ^"^  inch,  or  i^^ 
of  a  foot.  In  other  words,  the  Potomac  is  lowering  its 
hydrographic  basin  at  the  rate  of  one  foot  in  2772  years. 
Other  similar  estimates  are  given  in  the  table  on  page  75, 
which,  if  approximately  correct,  might  be  taken  as  indicat- 
ing the  work  that  the  rivers  of  the  world  are  doing.  It  is 
probable,  however,  that  except  in  the  case  of  the  Mississippi, 
in  the  table  <  ompiled  by  Geikie,  the  bottom  loads  of  the 
rivers  are  not  included.  I  am  also  inclined  to  doubt  the  ac- 
curacy of  some  of  the  other  measures  referred  to.  The 
average  for  the  nine  rivers  tabulated  is  one  foot  of  denuda- 
tion in  about  9000  years. 

Chemical  Degradation. — The  importance  of  the  slowly 
acting  and  invisible  process  by  which  the  surface  of  the  land 
is  being  lowered  by  solution,  has  only  recently  been  recog- 
nised. The  earliest  definite  discussion  of  the  rate  of  chemi- 
cal degradation  now  in  progress,  so  far  as  I  am  aware,  is  in 
a  series  of  three  papers  by  T.  Mellard  Reade.*     In  these 

'  A  comparatively  slight  discrepancy  comes  in  here,  since  the  specific  gravity 
of  the  bottom  load  and  of  the  silt  in  suspension  is  not  the  same. 

'  Republished  with  the  title,  Chemical  Denudation  in  Relation  to  Geolcgital 
Time.     Daniel  Dogue,  London,  1879. 


MATERIAL   CARRIED  BY  STREAMS 


83 


n 

>e 


instructive  essays  it  is  estimated  that  the  amount  of  material 
removed  in  a  century  by  the  streams  of  England  and  V^ales 
in  solution,  if  spread  evenly  over  the  land  from  which  it  is 
derived,  would  have  a  thickness  of  .oo'j'j  of  a  foot.  That  is, 
it  will  take  12,987  years  to  denude  the  surface  of  England 
and  Wales  of  one  foot  of  solid  matter  by  the  process  here 
considered,  under  the  supposition  that  the  material  is  taken 
evenly  from  all  parts  of  the  surface. 

The  Mississippi,  as  previously  stated,  carries  annually 
about  112,832,171  tons  of  mineral  matter  in  solution.  This 
amount  of  material  may  be  considered  as  about  equivalent 
to  1,350,000,000  cubic  teet  of  limestone,  and  if  spread  evenly 
over  the  Mississippi  basin  would  cover  it  to  the  depth  of 
about  73-5-inr  of  oiiG  foot ;  or,  in  other  words,  chemical 
degradation  is  lowering  that  area  at  the  rate  of  one  foot  in 
25,000  years.' 

Rate  of  Both  Mechanical  and  Chemical  Degradation. — The 
best  and  in  fact  the  only  approximately  reliable  measures 
we  have  of  the  rate  of  the  combined  mechanical  and  chemi- 
cal degradation  by  the  rivers  of  North  America,  is  in  the 
case  of  the  Mississippi.  On  account  of  the  large  size  of  the 
drainage  area  of  that  river  and  the  variety  of  rocks  forming 
its  surface,  as  well  as  the  diversity  of  climate  included  within 
its  border,  it  may  be  reasonably  assumed  to  represent  about 
the  average  rate  of  degradation  which  is  being  performed  by 
rivers  in  general.  ;    ^      •    'w  v    r^  ;       \ 

'  The  material  in  solution  is  taken  in  part  from  the  surface  and  in  part  from 
below  the  surface.  While  an  estimate  of  the  average  lowering  of  the  surface 
by  degradation  during  a  single  year  need  not  perhaps  include  the  material 
removed  in  solution  from  below  the  surface,  yet  this  should  certainly  be  taken 
into  account  in  estimates  of  average  degradation. 


||fe 


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the  process  of  mechanical  dejijraclation  at  the  rate  of  one 
foot  in  4638  years,  and  by  sokition  at  the  rate  of  one  foot 
in  25,000  years.  The  avcraj^e  rate  of  general  degradation 
is  therefore  about  one  foot  in  3912,  or,  in  round  numbers, 
4000  years. 

This  estimate  of  the  rate  at  which  the  Mississippi  is  lower- 
ing its  drainage  basin  is  somewhat  greater  than  is  given  in 
several  text-books  of  geology,  but  in  these  the  amount  of 
material  carried  in  solution  and  the  bottom  load  of  the 
river  do  not  seem  to  have  been  taken  into  account.  An 
inspection  of  the  tables  on  pages  74  and  75,  shows  that  the 
Mississippi  is  removing  material  from  the  land  at  a  less  rate 
than  is  the  case  with  several  other  rivers,  and  at  even  a  less 
rate  than  the  average  of  the  rivers  tabulated. 


UNDERGROUND   STREAMS 


The  water  which  finds  its  way  below  the  surface  of  the 
land  for  the  most  part  percolates  through  the  rocks  without 
forming  definite  streams.  When  the  rocks  are  readily 
soluble,  however,  as  when  limestone  is  present  in  thick 
layers,  underground  channels  are  frequently  dissolved  out, 
and  subterrarean  streams  occur  of  sufficient  size  to  be 
classed  as  rivers.  During  the  underground  flow  of  water, 
whether  percolating  through  porous  rocks  or  forming  streams 
in  caverns,  it  is  brought  into  contact  with  the  material 
forming  the  earth's  crust,  thus  facilitating  solution.  One 
marked  difference  between  surface  and  subterranean  streams 
is  that  the  former  peiform  the  task  of   eroding  the  land 


MATERIAL   CAKKIED  BY  STREAM H 


«5 


mainly  by  nicchiinical  mcan.-i,  while  the  latter  carry  on  a 
similar  work  principally  by  soUition.  Undcrj^round  streams, 
in  part,  occupy  ready-made  galleries  or  caverns,  like  the  open- 
ing along  fractures  and  fau)*:.i,  or  the  tunnels  in  lava  streams 
formed  by  the  flowing  out  of  the  still  molten  parts  after 
a  crust  formed.  More  frequently,  however,  svibterranean 
streams  make  passageways  for  themselves.  This  process 
is  analogous  to  the  excavation  of  the  valleys  by  surface 
streams.  We  may  carry  this  analogy  a  step  farther,  and 
say  that  the  underground  streams  flowing  through  pre- 
viou.sly  made  channels  are  subterranean  consequent-streams, 
and  those  which  dissolve  out  their  own  galleries  arc  sub- 
terranean subsequent-streams.  Ikit  there  is  little,  if  any, 
advantage  in  such  a  nomenclature. 

The  conditions  most  favourable  for  the  beginning  and 
growth  of  subterranean  streams  -^re  that  the  rocks  should  be 
of  comparatively  easy  solubility,  and  also  in  thick,  nearly 
horizontal,  and  unbroken  layers,  situated  at  a  greater  eleva- 
tion than  the  adjacent  surface  valleys.  The  rocks  most 
easily  dissolved  by  cold  water,  and  the  ones,  too,  which 
frequently  occur  in  thick,  nearly  horizontal  layers,  are  the 
limestones.  For  this  leason  most  caverns  arc  in  such  beds. 
If  the  soluble  layer  has  a  bed  of  less  soluble  rock  in  contact 
with  it  both  above  and  below,  the  conditions  for  the  pro- 
duction of  large  caverns  are  still  more  favourable. 

The  advantage  of  having  the  soluble  layer  elevated  above 
adjacent  surface  drainage  is  that  the  water  flowing  through 
the  galleries  opened  in  it  may  readily  escape  and  flow 
rapidly.  A  roof  of  rock  which  doec  not  yield  readily  to  the 
solvent  action  of  percolating  water  admits  of  the  dissolving 


'J 


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' 


1 

1 

HW 

86 


RIVERS  OF  NORTH  AMERICA 


out  of  broad  galleries  beneath  and  decreases  the  liability  of 
their  roofs  to  fall.  A  roof  to  a  natural  cavern  is  as  essential 
as  a  rocf  to  a  mine. 

Subterranean,  like  surface  streams  are  dependent  on  rain- 
fall for  their  water  supply.  They  are  also  influenced  by 
other  climatic  conditions.  A  humid  climate  not  only  in- 
sures an  abundant  water  supply,  but  favours  the  growth  of 
vegetation,  which  contributes  organic  acids  to  the  descend- 
ing waters,  thus  increasing  their  power  to  dissolve  mineral 
substances.  A  dry  climate  not  only  decreases  the  water 
supply  of  subterranean  streams,  but,  as  will  be  shown  later, 
favours  the  precipitation  of  mineral  matter,  principally 
calcium  carbonate,  in  pre-existing  rock-openings. 

The  temperature  element  of  climate  also  plays  a  part  in 
the  history  of  underground  drainage.  A  warm  climate,  if 
humid,  insures  a  luxuriant  vegetation,  and  hence  an  abund- 
ant supply  of  organic  acids,  while  a  dry  climate  has  a  reverse 
influence.  A  cold  climate,  by  leading  to  the  freezing  of  the 
water  in  soils,  checks  percolation,  and  in  other  ways  exerts 
an  unfavourable  influence  on  the  tendencies  of  sub-surface 
waters  to  enlarge  the  galleries  they  flow  through. 

Underg'-ound  passages  in  many  instances  owe  their  incep- 
tion to  joints  and  fractures  of  small  width  which  arc  enlarged 
by  solution.  The  percolating  of  water  through  porous 
rocks,  however,  on  account  of  inequalities  in  rock  texture 
or  composition,  might  be  freer  along  certain  courses  than 
along  others,  and  thus  lead  to  unequal  waste  by  solution, 
and  to  the  making  of  cavities  and  galleries.  When  once  a 
beginning  is  made,  from  whatever  cause,  the  flowing  waters 
tend  to  enlarge  the  galleries  they  pass  through  by  dissolv- 


MATERIAL    CARRIED  BY  STREAMS 


87 


ing  their  walls.  The  water  which  finds  its  way  through 
such  galleries  eventually  emerges  as  springs,  or  again 
reaches  the  surface  by  percolation.  The  springs  supplied 
from  caverns  usually  come  to  the  light  in  the  sides  or  bot- 
toms of  valleys,  and  contribute  their  water  to  the  surface 
drainage;  but  not  infrequently  they  emerge  at  the  bottoms 
of  lakes,  or  even  beneath  the  sea. 

Subterranean,  like  surface  streams  remove  rock  material 
both  by  mechanical  and  chemical  means,  but  the  relative 
importance  of  the  two  processes  is  reversed.  Caverns  are 
enlarged  principally  by  solution,  but  mechanical  wear  is 
frequently  and  sometimes  an  important  part  of  the  process. 
The  flow  of  water  through  smaU  or  tortuous  passages,  the 
beginning  of  caverns,  is  probably  sluggish  in  most  instances, 
thus  favouring  solution,  but  retarding  mechanical  corrasion. 
Water,  in  descending  to  small  underground  passages,  mostly 
percolates  through  soil  or  rock  debris,  and  is  thus  filtered. 
This,  agam,  is  unfavourable  for  mechanical  abrasion,  since 
solid  particles,  the  tools  with  which  flowing  water  performs 
the  greater  part  of  its  mechanic:il  work,  are  removeu.  As 
underground  galleries  become  larger,  and  especially  when 
the  surface  of  the  country  is  lowered  by  denudation,  open- 
ings in  their  roofs  are  frequently  made,  into  which  surface 
streams  plunge  with  all  of  their  freight  of  material  in  sus- 
pension. The  waters  in  the  underground  courses  of  such 
streams  are  more  or  less  heavily  charged  with  sediment, 
and  mechanical  corrasion  assists  in  the  enlargement  of  the 
passageways  they  flow  through.  The  rocks  removed  in 
order  to  make  subterranean  galleries  frequently  contain 
sand   grains,  chert  nodules,  silicified  fossils,  etc.,  of  diflfi- 


:#i 


I 


mmmmi 


■pm 


—mil 


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m 


88 


RIVERS  OF  NORTH  AMERICA 


cult  solubility.  Such  bodies  are  separated  by  the  re^ 
moval  of  the  more  soluble  material  enclosing  them,  and, 
being  swept  along  by  the  streams,  assist  the  debris 
brought  from  above  in  abrading  the  rocks  with  which  the 
waters  are  brought  in  contact.  The  behaviour  of  under- 
ground streams  is  thus  seen  to  be  similar  in  many  ways  to 
surface  streams.  As  will  appear  as  we  proceed,  cavern 
streams  not  only  remove  rock  material,  but  also  make  both 
mechanical  and  chemical  deposits,  thus  still  further  bearing 
out  their  similarity  to  ordir  i:y  brooks  and  rivers. 

The  features  of  subterranean  streams  just  enumerated, 
and  of  the  results  they  bring  about,  are  illustrated  in  the 
well-known  instance  of  Mammoth  Cave,  Kentucky.  The 
surface  rocks  in  that  region  are  mainly  horizontally  bedded 
limestones,  about  350  feet  thick.  Above  the  limestone 
there  is  a  sheet  of  sandstone,  which  forms  the  roof  of  the 
higher  series  of  galleries.  The  ground  above  the  cavern, 
over  an  area  of  thousands  of  acres,  is  elevated  from  300  to 
350  feet  above  the  adjacent  valley  of  Green  River,  and  is 
mostly  without  surface  streams.  The  rain-water  is  largely 
absorbed  by  the  deep  residual  soil  left  by  the  solution  of 
hundreds  of  feet  of  limestone,  and  percolates  slowly  down- 
ward to  the  galleries  beneath.  During  summer,  all  of  the 
water  entering  the  cavern  probably  gains  access  in  this  mea- 
ner, but  during  heavy  rains,  surface  lills  and  brooks  aie 
formed  which  plunge  into  openings  or  "  sink-holes  "  and 
enter  the  underground  galleries  directly  as  heavily  silt- 
laden  streams.  After  rains,  as  the  writer  has  observed, 
the  sti earns  within  the  cavern  are  muddy,  flow  swiftly  in 
many  places,  and  are  actively  engaged  in  mechanical  as  well 


MATERTAL   CARRIED  BY  STREAMS 


89 


as  chemical  corrasion.  The  manner  in  which  Echo  River, 
the  largest  stream  in  Mammoth  Cave,  is  now  enlarging  its 
channel  is  much  the  same  as  in  the  case  of  many  surface 
streams,  except  that,  owing  to  the  irregularities  and  espe- 
cially the  numerous  constrictions  in  the  passages  it  follows, 
its  waters,  when  in  flood,  '^--e  in  contact  with  the  roofs  of 
the  galleries  in  some  localities,  and  corrasion  occurs  above  as 
well  i- :  at  the  bottom  and  sides.  Like  many  surface  streams, 
Echo  Ri^^er  has  its  rapids,  cascades,  and  quiet  reaches,  and 
in  places,  also,  is  ponded  and  forms  lakelets.  A  photo- 
graph of  this  remarkable  river  is  reproduced  in  Plate  IV. 
Applying  the  criteria  to  be  described  later,  by  means  of 
which  young  surface  streams  may  be  distinguished  from 
mature  or  well-adjusted  streams,  to  this  example  of  subterra- 
nean drainage,  we  find  it  to  be  but  imperfectly  adjusted  to  its 
environmenc,  and,  therefore,  it  may  be  designated  as  an  im- 
mature subterranean  river.  It  has  not  deepened  its  channel 
throughout  to  the  level  of  Green  River  into  which  it  dis- 
charges, and  which  determines  the  baselevel  of  subterranean 
drainage  in  the  rocks  forming  the  adjacent  uplands.  As  is 
the  rule  with  surface  "streams,  the  adjustment  of  Echo  River 
to  baselevel  has  progressed  most  rapidly  in  its  lower  course. 
Near  where  it  comes  to  the  light  as  a  huge  spring  on  the 
border  of  Green  River,  it  is  about  on  a  level  with  that 
stream.  Farther  within  the  cavern,  it  has  many  high-grade 
reaches,  and  is  fed  by  torrent-like  tributaries.  A  farther 
advance  in  its  life  history,  the  present  local  baselevel  being 
maintained,  should  be  characterised  by  the  development  of 
features  analogous  to  those  of  a  broad  valley.  A  great 
gallery,  corresponding  with  the  valley  of  a  surface  stream, 


tJ> 


SI 

m 


;  '■ 


90 


RIVERS  OF  NORTH  AMERICA 


^ 

1 

should  be  formed  with  its  floor  about  on  a  level  with  the 
adjacent  portion  of  Green  River. 

The  lowering  of  Green  River  has  led  to  the  deepening 
of  the  channels  of  tributary  underground  streams,  and  the 
abandonment,  as  avenues  of  drainage,  of  many  galleries 
that  were  formerly  waterways.  The  occurrence  of  one 
series  of  galleries  above  another,  or  the  origin  of  the  several 
stories  in  the  cavern-house,  as  Mammoth  Cave  may  be 
termed,  can  be  accounted  for,  in  part  at  least,  on  the  prin- 
ciple just  referred  to:  the  highest  series  of  galleries  having 
been  formed  at  a  time  when  Green  River  flowed  at  the 
level  of  their  place  of  discharge,  and  each  lower  series  dis- 
solved out  during  subsequent  stages  in  the  deepening  of 
master  rivers. 

A  different  explanation  from  that  just  suggested  has  been 
advanced  by  N.  S.  Shaler,'  who  states  that  the  floors  of  the 
various  stories  in  the  great  cavern  are  formed  of  resistant 
layers,  each  of  which  gave  a  lateral  direction  to  the  flowing 
water,  until  an  opening  was  found  leading  to  the  next  layer 
of  limestone  below,  when  the  process  of  lateral  excavation 
was  repeated.  Hard  layers,  or  layers  less  pervious  than 
those  above  and  below,  have  had  an  important  influence  in 
the  development  of  the  cavern,  but  rather,  it  seems  to  me, 
in  the  direction  of  modifying  the  action  of  the  streams  in 
response  to  a  lowering  of  the  place  of  discharge,  than  in 
furnishing  the  main  control.  The  adjustment  of  under- 
ground streams  to  rock  texture  is  analogous  to  the  adjust- 
ment of  surface  streams  to  the  conditions  furnished  by  hard 
and  soft  rocks. 

"^  Aspects  of  the  Earth,  p.  109.     Scribner's  Sons,  1889.  — 


MATERIAL    CARRIED  BY  STREAMS 


91 


A  surface  "stream,  as  the  reader  has  learned,  when  carry- 
ing debris  but  not  overloaded,  corrades  its  channels;  but  if 
j^  its  velocity  is  checked,  will  deposit  a  part  or  all  of  its  load. 

These  same  features  characterise  subterranean  streams. 
When  subterranean  waters  run  swiftly  they  flow  over  bare 
rock,  but  when  their  velocities  are  checked,  sediment  is  de- 
posited. Many  of  the  galleries  in  Mammoth  Cave  and 
other  similar  caverns  have  long  since  been  abandoned  as 
avenues  of  drainage,  and  are  deeply  filled  with  what  is 
t'^'^med  "  cave  earth."  In  part,  this  material  has  been 
brought  into  the  cavern  by  streams  from  the  surface,  but  to 
some  extent,  certainly,  it  is  the  residue  left  by  the  solution 
of  limestone.  Ordinary  grey  limestone  contains  about  one 
per  cent,  of  insoluble  material  which  remains  when  the 
calcium  carbonate  is  removed,  and  forms  a  reddish  clay. 
The  sedimentary  deposits  in  caverns  are  frequently  terraced  ; 
showing  that  the  streams  after  dropping  their  burden  of  silt 
have  been  able  to  again  resume  the  work  of  transportation 
anJ.  to  cut  channels  through  it.  Conditions  favouring  sedi- 
mentation are  frequently  brought  about  during  high-water 
stages  when  the  water  in  certain  galleries  is  ponded,  owing 
to  constriction  lower  down  their  course.  Subsequently, 
during  low-water  stages,  the  streams  are  not  hv^Id  in  check, 
and  resume  the  task  of  deepening  their  channels.    ,  • 

In  dwelling  on  the  similarity  between  the  mechanical 
action  of  surface  and  of  sub-surface  streams,  I  do  not  wish 
to  be  understood  as  advocating  the  view  that  caverns  are 
enlarged  principally  by  the  friction  of  the  debris  !n  the 
water  flowing  through  them ;  the  main  agency  in  their 
growth  in  most,  and  possibly  all,  instances  is  solution. 


■m 


m 


92 


HI  VERS  OF  NORTH  AMERICA 


There  is  a  process  in  the  chemical  action  of  subterranean 
streams  analogous  to  the  manner  in  which  they  mechanically 
corrade  and  deposit  alternately.  When  the  waters  are 
abundant  and  flow  freely,  they  carry  away  all  of  the  material 
contributed  to  them  in  solution,  and  passageways  are  en- 
larged. When,  however,  the  water  supply  is  greatly  di- 
minished, and  water  falls  from  the  cavern  roofs  mainly  in 
drops,  or  descends  their  sides  and  flows  over  their  floors 
in  thin  sheets,  evaporation  leads  to  an  increase  in  the  per- 
centage of  mineral  matter  in  solution  and  to  the  precipitation 
of  certain  sr  Us. 

In  addition  to  the  evaporation,  when  the  mineral-charged 
waters  fall  in  drops,  or  are  spread  out  in  thin  sheets,  and  of 
more  importance  in  the  history  of  caverns,  is  the  fact  that 
these  conditions  lead  to  the  escape  of  carbonic  acid.  Lime 
(calcium  carbonate),  the  most  abundant  substance  dissolved 
by  both  surface  and  cavern  waters,  is  held  in  solution  owing 
to  the  presence  of  carbonic  acid,  and  is  precipitated  when  it  is 
removed.'  The  calcium  carbonate  precipitated  on  the  roofs 
of  caverns  frequently  takes  pendent  or  icicle-like  forms, 
termed  stalactites.  After  the  water  falls  to  the  floor  of  the 
caverns  precipitation  is  continued,  and  sheets  and  pillars  of 
calcium  carbonate,  termed  stalagmites,  are  formed. 

Calcium  carbonate  is  by  far  the  most  abundant,  and,  in 
fact,  in  most  caverns,  seems  to  be  the  only  precipitate 
thrown  down.     The  roofs  of  caverns,  however,  are  some- 

'  Pure  water  when  cold  dissolves  calcium  carbonate  in  the  proportion  of  one 
part  of  the  salt  to  10,800  parts  of  water,  and  8875  parts  if  the  water  is  boiling 
(Fresenius) ;  but  if  charged  with  carbon  dioxide  cold  water  will  dissolvs  one 
part  in  about  1000  :  the  calcium  when  in  solution  is  in  the  condition  of  a 
bicarbonate. 


I 


MATERIAL    CARRIED  BY  STREAMS 


93 


times  beautified  by  rosettes  and  star-like  brilliants  of  gypsum 
(calcium  sulphate)  and  by  other  and  more  soluble  salts 
which  appear  as  crystalline  efflorescences. 

It  is  this  process  of  lining  caverns  with  crystalline  in- 
crustations, the  forming  of  pendants  of  many  shapes  and 
tints  from  their  roofs,  as  well  as  of  no  less  beautiful  and 
frequently  grotesque  stalagmite  columns,  that  gives  to  those 
silent  galleries  of  the  nether  world  much  of  their  fascination 
and  beauty.  One  of  the  very  finest  examples  of  subter- 
ranean galleries,  partially  filled  by  calcareous  precipitates 
from  percolating  waters,  is  furnished  by  the  beautiful  Luray 
Cavern,  Virginia.  Other  remarkable  illustrations  of  the 
same  occurrence  may  be  seen  in  the  justly  celebrated 
Wyandotte  Cavern,  Indiana. 

The  process  of  infiltration,  just  noticed,  if  allowed  to 
continue,  will,  in  time,  fill  the  cavern  previously  excavated 
when  the  water  supply  was  abundant.  In  many  caverns 
galleries  occur  which  have  been  nearly  closed  by  this 
method,  and  in  other  instances  it  is  evident  that  the  filling 
has  been  completed.  Checks  in  the  process  of  refilling  may 
evidently  occur  from  an  increase  in  water  supply  or  from  its 
nearly  complete  cessation.  Surface  changes,  such  as  the 
removal  of  vegetation,  would  also  influence  the  conditions 
favouring  enlargement,  or  refilling,  in  the  caverns  beneath. 

Many  peculiarities  in  the  surface  features,  particularly  of 
limestone  regions  where  caverns  occur,  have  an  intimate 
relation  to  the  galleries  beneath.  The  boles  through  which 
surface  water  descends  are  enlarged  by  solution  or  by  the 
falling  of  portions  of  the  roofs  of  the  caves,  and  become 
basins  termed  "  sink-holes."     Should  the  openings  in  the 


li  ' 


■* 


yjii 

1 

'111 
III 


94 


mVEKS  OF  NORTH  AMERICA 


bottom  of  these  depressions  become  filled,  lakelets  may  be 
formed.  Such  basins  and  lakelets  are  a  characteristic  feature 
of  many  limestone  regions,  as,  for  example,  in  the  Great 
Appalachian  valley,  Western  Kentucky,  and  much  of  Ten- 
nessee. Some  of  the  basins  near  Mammoth  Cave,  caused 
by  the  solution  of  the  rocks  beneath  and  the  falling  of  the 
roofs  of  caverns,  occupy  an  area  of  about  2000  acres.* 

The  subsidence  of  the  roofs  of  caverns  also  gives  origin 
to  trench-like  depressions  in  the  surface  above,  which  be- 
come avenues  of  drainage.  The  course  of  Green  River  near 
Mammoth  Cave,  is  through  a  depression  of  this  character 
which  has  been  modified  by  surface  erosion.  When  portions 
of  a  cavern  roof  fall,  leaving  other  portions  in  position, 
natural  bridges  and  tunnels  result.  The  most  striking 
example  of  this  nature  is  the  justly  famous  Natural  Bridge 
of  Virginia. 

Another  but  minor  influence  of  subterranean  drainage 
on  the  relief  of  the  region  above,  is  brought  about  when  the 
caverns  by  conducting  away  the  surface  waters  decrease 
the  rate  of  surface  erosion.  When  this  occurs  and  the 
lowering  of  the  adjacent  land  continues,  the  rocks  traversed 
by  galleries  are  left  in  relief  and  form  a  mound  or  perhaps 
a  series  of  hills,  while  the  surrounding  country,  without 
sub-drainage,  sinks  into  valleys.  An  illustration  of  this  is 
furnished  by  the  low  hills  in  which  Luray  Cavern  is  located. 

Subterranean  drainage  goes  on  with  the  greatest  freedom 
and  produces  the  most  conspicuous  results  above  the  level 
of  adjacent  surface  streams.     This  assertion  needs,  perhaps. 


'  Hovey  and  Call,  The  Mammoth  Cave  of  Kentucky,  p.  4. 
Louisville,  1897. 


Morton  &  Co., 


MATERIAL    CARRIED  BY  STREAMS 


95 


to  be  qualified,  as  only  such  caverns  as  are  above  the  level 
of  neighbouring  valleys  are  ordinarily  open  to  inspection, 
while  the  size  and  extent  of  the  galleries  below  the  level  of 
surface  drainage  car  only  be  judged  from  indirect  evidence. 
The  flew  of  water  through  caverns  above  the  level  of  the 
surface  streams  into  which  they  discharge,  except  in  part 
during  high-water  stages,  is  due  directly  to  gravity — that  is, 
the  waters  flow  as  through  ordinary  open  channels;  but  the 
flow  through  lower  galleries  is  produced  by  hydraulic  pres- 
sure, and  is  similar  to  the  movement  of  water  through  pipes, 
as  in  the  case  of  the  water-mains  of  a  city.'  There  is  a 
marked  difference  in  these  two  methods  in  reference  espe- 
cially to  mechanical  corrasion.  The  work  of  subterranean 
streams  in  galleries  not  completely  filled  is,  in  most  in- 
stances, mainly  in  the  direction  of  solution,  but  is  aided  also 
by  the  friction  of  particles  in  suspension  or  rolled  along  the 
bottom.  In  such  instances,  also,  the  conditions  are  nor- 
mally unfavourable  for  sedimentation.  When  water  is 
forced  through  galleries  by  hydraulic  pressure,  they  are 
completely  filled  up  to  the  level  of  the  surface  of  the  re- 
servoir, friction  is  greatly  increased,  and  the  rate  of  flow  is 
normally  not  rapid.     This  is  an  incomplete  statement,  to  be 


'        t*t   *  ,01' 


'  The  possible  influence  of  heat  in  causing  water  to  flow  through  subterranean 
galleries  has  recently  been  discussed  by  F.  W.  Crosby  and  VV.  O.  Crosby 
{Technology  Quarterly,  vol.  ix.,  pp.  6-23,  Boston,  1896)  in  connection  with  a 
study  of  what  are  known  as  the  >Sea  Mills  of  Cephalonia,  in  Greece.  In  this 
instance  fresh  water  at  the  ordinary  surface  temperature  enters  openings  at 
sea-level  and  is  supposed  to  emerge  below  sea-level  and  against  the  pressure 
of  the  denser  sea-water.  In  explanation  of  this  exceptional  occurrence,  it  is 
suggested  th  it  the  descending  water  traverses  a  more  or  less  U-shaped  system 
of  galleries,  and  is  heated  in  the  ascending  portion  of  its  course  and  thus  rend- 
ered lighter  than  the  inflowing  and  colder  water. 


r; 


96 


JKIVERS  OF  NORTH  AMERICA 


iiil 


II 


sure,  but  sufficient,  I  think,  to  show  that  below  surface 
drainage  the  conditions  ire  less  favourable  for  cavern-mak- 
ing than  in  similar  rocks  situated  at  higher  levels.  In 
galleries  below  surface  drainage,  it  may  be  reasonably  pre- 
sumed, mechanical  corrasion  is  retarded  and  the  conditions 
favouring  sedimentation  are  augmented. 

Deep  within  the  earth's  crust  the  conditions  differ  from 
those  near  the  surface.  The  tendency  of  pressure  to 
close  cavities  increases  with  depth.  The  limit  below  which 
openings  of  such  size  as  to  be  classed  as  caverns  can 
exist,  must  be  at  a  very  modeiate  depth, — possibly  not 
more  than  a  few  thousand  feet.  With  increasing  depth, 
also,  there  is  a  progressive  rise  of  temperature  \^aO  far  within 
the  earth  as  man  has  ever  penetrated),  which  exerts  a 
marked  influence  on  the  solvent  power  of  water.  A  rise 
of  temperature,  at  least  until  excessive  heat  is  reached,  in- 
creases the  solvent  power  of  water  for  all  common  minerals 
except  calcium  carbonate.  With  the  increase  of  temperature^ 
and  of  pressure,  in  subterranean  waters,  there  is  an  increase 
in  the  variety  and  abundance  of  mineral  substances  which 
are  taken  into  solution,  and  also  increased  chemical  reac- 
tions, some  of  which  leac  lo  precipitation  and  to  the  filling 
of  openings.  These  ard  ;  dll  other  reasons  favour  the  belief 
that  caverns,  and  consequently  underground  streams,  do 
not  exist  below  an  extremely  superficial  portion  of  the 
earth's  crust.' 

'  To  the  books  of  reference  concerning  American  caverns  already  mentioned, 
I  would  add  the  following  : 

H.  C.  HOVEY.     Celebrated  American  Caverns.     R.  Clarke,  Cincinnati,  1882. 

\V.  S.  Blatchley,  "  Indiana  Caves  and  their  Fauna,"  in  Geological  Sur- 
vey of  Indiana,  Twenty-first  Annual,  Report,  pp.  121-212,  1896. 


CHAPTER  V 


STREAM  DEPOSITS 


i  » 


\ 


THE  ability  of  a  stream  to  carry  detritus  in  suspension, 
as  we  have  seen,  varies  as  the  sixth  power  of  its 
velocity.  The  velocity  depends  mainly  on  the  steepness  of 
the  stream  channel  and  on  the  volume  of  water.  It  is  a 
familiar  fact  that  streams  vary  in  velocity  throughout  their 
courses.  In  general,  mainly  as  a  result  of  development, 
their  channels  are  steepest  and  their  waters  swiftest  near 
their  sources,  and  become  less  steep  and  more  sluggish  as 
they  near  the  sea;  but  throughout  their  length  they  are 
commonly  broken  into  alternate  swift  and  quiet  reaches 
or  sections.  Evidently  a  stream  may  be  able  to  transport 
ail  of  the  debris  brought  to  it  in  certain  portions  of  its 
course,  and  to  corrade  its  channel,  while  in  other  portions 
it  ma>r  be  overloaded  and  consequently  forced  to  drop 
a  portion  or  even  all  of  the  material  previously  held  in 
suspension.  Evidently,  then,  streams  both  transport  and 
deposit  material  as  a  part  of  their  normal  work.  It  i?  to  the 
manner  in  which  streams  lay  down  their  burdens  and  to  the 
character  and  histories  of  the  deposits  thus  formed  that 
attention  is  now  invited. 

Many  streams,  and  especially  large  rivers,  rise  in  mount- 

97 


98 


RIVERS  OF  NORTH  AMERICA 


f ,. ,, 


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i . 

ains  and  flow  across  broad  plains  to  the  sea.  It  is  con- 
venient to  divide  such  streams  into  three  sections  in 
accordance  with  their  velocities  and  with  reference  also  to 
the  topography  of  their  borders.  Although  these  divisions 
are  really  determined  by  the  stage  of  development  each 
stream  has  reached  in  various  portions  of  its  course,  they 
are  somewhat  definite  for  such  periods  of  time  as  man  may 
have  been  acquainted  with  them.  The  three  divisions  re- 
ferred to  are  the  mountain  tract,  where  the  waters  usually 
flow  impetuously  in  narrow,  trench-like  depressions;  the 
valley  tract,  where  a  stream  widens  and  is  bordered  by  nar- 
row flood-plains;  and  the  plains  tract,  where  the  grade  is 
still  more  gentle,  and  the  stream  meanders  in  broad  curves 
through  alluvial  lands  of  its  own  making.        •    :  ■. 

In  the  mountains  the  streams,  as  a  rule,  are  swift,  and 
able  to  bear  along  not  only  fine  debris  in  suspension,  but 
to  roll  great  boulders  down  their  channels,  especially  during 
high- water  stages.  In  the  valleys  through  which  the  streams 
leave  the  mountains,  the  current  slackens,  and  the  coarser 
material  brought  from  above  is  dropped.  In  the  plains  tract 
velocity  is  again  diminished,  and  again  "  "election  is  made, 
the  coarser  portions  of  the  burdens  brought  from  above 
being  deposited,  and  only  the  finest  silt  and  sand  ordinarily 
carried  forward. 

The  cropping  of  material  in  the  valley  and  plains  tracts 
of  a  stream  leads  to  the  filling  in,  or  aggrading,  of  its 
channel  in  those  parts.  This  means  an  increase  in  the  gen- 
eral gradient  of  the  stream  channel  below  the  mountain 
tract  and  consequently  a  swifter  flow  and  an  increase  in  the 
transporting  power  of  the  current.     Coarse  material  is  then 


■ 

L 


Plate  V. 


lets 

its 

jen- 

Itain 


tl 


ic 


Lhtu 


iJ* 


Delta  of  the  Mississippi  ;  by  U.  S.  Coast  Survey. 
SoundinitR  on  tlntted  iire.i»  In  feet,  all  other»  in  f^thumt. 


■frni^pi 


HAS' 


STREAM  DEPOSITS 


99 


carried  farther  than  before  the  stream  bed  was  raised,  and  a 
new  adjustment  is  made  throughout  the  valley  and  plains 
tract.  This  process  of  aggrading  will  continue  as  long  as 
the  material  brought  by  the  swift  head-waters  of  a  stream  is 
in  excess  of  the  transpcting  power  of  the  current  lower  ^ 
down.  As  will  be  shown  later,  a  stream  throughout  much 
of  its  life  deepens  its  channel  in  its  mountain  tract  and  de- 
posits all  but  the  finest  of  the  material  thus  removed  in  the 
lower  and  less  swift  portion  of  its  course. 

The  action  of  a  stream  in  corrading  its  channel  in  one 
portion  of  its  course  and  aggrading  it  in  another  portion,  is 
carried  on  at  the  same  time,  and  is  a  highly  complex  pro- 
cess. This  complexity  is  again  increased  especially  by 
variations  in  the  volumes  of  the  streams.  During  high- 
water  stages  a  stream  moves  more  material  and  carries  it 
farther  than  during  low-water  stages.  The  result  of  these 
varying  conditions  may  be  seen  in  any  stream  that  rises  in 
high  lands,  and  flows  through  a  valley  and  across  a  plain.  In 
the  hill  or  mountain  tract  the  stream  channel  is  narrow  with 
steep  sides,  and  its  bed  comparatively  free  of  fine  debris, 
except  in  crevices  and  holes,  although  perhaps  clogged  with 
p  2at  stones  tco  large  for  the  water  to  move  except  during 
u;iies  of  unusual  flood.  In  the  valley  tract,  where,  for 
reasons  to  be  considered  later,  the  gorge  through  whicii  the 
stream  flows  becomes  wider,  the  stream  bed  is  occupied  by 
coarse  gravel  and  small  boulders,  and  much  fine  material 
may  be  found  in  the  more  sluggish  reaches.  Where  the 
stream  emerges  into  a  plain,  its  bed  is  lined  with  fine  sand 
an  1  silt,  except  in  the  swifter  reaches,  where  gravel  occurs. 

This  general  decrease  in  the  size  of  the  debris  dropped 


'IS 


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by  streams  between  their  sources  and  their  mouths  would 
be  much  more  regular  than  one  ordinarily  finds  it,  if  it  were 
not  for  the  fact  that  velocity  increases  with  volume,  other 
conditions  remaining  the  same,  so  that  coarser  material  is 
carried  during  floods  to  localities  where  only  fine  material  is 
normally  dropped.  As  a  rule,  streams  increase  in  volume 
from  their  sources  to  their  mouths,  but  the  increase  in 
velocity  due  to  this  cause  is,  in  most  cases,  more  than 
counterbalanced  by  loss  of  grade. 

The  debrij  chat  starts  from  the  mountains  on  its  journey 
down  a  stream  channel  to  the  sea  makes  I'^any  halts,  and  is 
gradually  reduced  in  size  both  by  mechanical  wear  and  by 
solution.  The  periods  of  rest  are  due  largely  to  variations 
in  the  volume  of  the  stream,  and  also  to  its  general  decrease 
in  grade  from  source  to  mouth.  The  larger  stones  are  de- 
layed until  finer  debris  washed  against  them  and  the  solvent 
power  of  the  water  reduce  their  size  so  that  the  stream  is 
competent  to  move  them.  The  material  laid  aside  is  con- 
stantly undergoing  chemical  changes  which  decrease  the 
size  of  the  fragments  by  solution  and  tend  to  make  them 
friable,  so  that  they  are  more  easily  broken  or  worn  when 
next  exposed  to  mechanical  agencies.  Even  during  the 
more  continuous  portions  of  the  journey  of  stream-carriet' 
debris,  its  onward  movement  is  irregular,  since  particles 
carried  upward  by  ascending  currents  are  pulled  down  by 
gravity,  and  reaching  the  bottom  may  rest  for  a  time  until 
disturbed  and  again  carried  upward  so  as  to  feel  the  influ- 
ence of  the  general  onward  flow.  For  these  and  other 
reasons,  the  journeys  of  the  debris  transported  by  rivers  arc 
varied  and  usually  long  delayed.     The  removal  of  a  given 


where 


STREAM  DEPOSITS 


lOI 


, 


rock  fragment  from  the  mountains  to  the  sea  may  require 
tens  of  thousands  and  even  millions  of  years.  The  drop- 
ping of  debris  by  streams,  or  its  lying  aside  until  conditions 
for  transportation  are  more  favourable,  leads  to  the  origin 
of  several  varieties  of  deposits. 


ties 

by 


Iher 


ALLUVIAL   CONES  :  N 

The  influence  of  a  decrease  of  slope  in  a  stream's  bed  is 
revealed  by  deposits  of  debris  in  nearly  every  stream,  but 
finds  its  greatest  compression  when  sudden  and  excessive 
changes  from  high  to  low  grade  occur.  When  a  stream  de- 
scends a  precipitous  gorge  in  a  mountain-side  and  emerges 
into  a  valley,  there  is  an  abrupt  loss  of  power,  and  the 
greater  part  or  perhaps  all  of  the  load  that  the  swift  waters 
in  the  mountain  tract  swept  along,  is  deposited.  A  conical 
pile  of  debris  is  thus  formed,  the  base  of  which  is  in  the 
valley  and  the  apex  at  ♦^he  mouth  of  the  high-grade  gorge 
through  which  it  was  swep*;  out  of  the  mountains.  (See 
Plate  VI.)  In  America  such  piles  of  debris  are  termed  allu- 
vial cones.  In  India '  and  in  Europe  they  are  generally 
known  as  alluvial  fans,  in  reference  to  their  fan-like  forms 
when  seen  from  above.  As  suggested  by  Gilbert,  there  is 
an  advantage  to  be  gained  by  retaining  both  of  these  terms, 
employing  cone  when  the  angle  of  slope  is  high,  and  fan 
when  it  is  low. 

The  best  example  of  alluvial  cones  occurs  in  arid  regions 
where  precipitous  mountains  border  desert  valleys.     They 

'  An  excellent  account  of  alluvial  fans  in  India,  by  Frederick  Drew,  may  be 
found  in  the  OuarUrly  Journal  of  tht  Geological  Society  of  London,  vol.  xxix., 
pp.  441-471,  1873- 


m 


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;  'i 


102 


RIVERS  OF  NORTH  AMERICA 


are  a  common  and  characteristic  feature  in  the  sceiiery  of 
tlie  arid  pc/tions  of  the  United  States  and  Mexico,  and  are 
especially  well  displayed  in  Utah,  Nevada,  and  Southern 
California.  In  that  region  the  intermittent  streams  from 
the  mountains  are  swollen  during  the  infrequent  storms  and 
sweep  along  large  quantities  of  debris.  When  the  waters 
reach  a  valley  and  the  grade  of  the  stream  abruptly  de- 
creases, they  lose  velocity  and  much  of  their  loads  is 
dropped.  In  many  instances,  the  waters  on  leaving  the 
mountains  where  their  channels  are  in  solid  rock  zvA  enter- 
ing alluvial-filled  valleys  are  absorbed  and  percolate  away. 
When  this  occurs,  all  of  the  debris  brought  from  the  mount- 
ain is  deposited.  Piles  of  debris  are  thus  formed  which 
are  of  all  sizes  up  to  one  or  two  thousand  feet  or  more  in 
height,  and  with  bases  frequently  three  to  five  miles  in 
radius.  The  slope  of  the  surfaces  of  these  piles  varies  with 
their  size,  the  grade  of  the  stream  that  deposited  them,  the 
nature  of  the  material  of  which  they  are  composed,  and 
possibly  other  conditions.  Their  profiles  along  lines  radiat- 
ing from  their  apexes  are  gently  convex  to  the  sky.  The 
material  of  which  they  are  composed  varies  in  size  from  fine 
silt  to  boulders  six  or  eight  feet  or  more  in  diameter  and 
weighing  hundreds  of  tons,  but  in  general  they  are  com- 
posed of  gravel  and  sub-angular  stones  and  fine  yellowish 
silt-like  material.  As  the  piles  increase  in  size  their  bases 
expand  and  their  apexes  are  extended  farther  and  farther 
up  the  feeding  gorges. 

In  the  arid  portions  of  North  America  referred  to,  alluvial 
cones  have  been  formed  in  such  abundance  and  of  such 
great  size  at  the  »nouths  of  canyons  on  the  borders  of  desert 


-|..,,; 


The 


Plate  VI. 


Fig.  a.     Sketch  of  Alluvial  Cones. 


I 


••I 


1 

i 


Kic;.  H.     Indian  Creek,  near  'IVyiorsville,  California. 

The  creek  meanders  through  a  flood-plnin  ;  rut  bank  on  the  left  in  the  foreitround,  and  a 
sloping  deposit  of  gravel  on  the  opposite  side  of  the  stream  ;  these  conditions  are 
reversed  at  (he  second  bend. 


! 

1 

1 

1 

L 

n. 

1 

STREAM  DEPOSITS 


103 


ranges,  that  they  unite  laterally  in  the  valleys  so  as  to  form 
a  fringe  about  the  bases  of  the  mountains.  The  contrast 
between  these  smooth,  sloping  pediments  and  the  angular 
crags  and  peaks  rising  above  them  is  frequently  very  marked, 
and  adds  an  interesting  feature  to  the  peculiar  scenery  of 
the  regions  where  they  are  best  developed.  The  bases 
of  the  united  alluvial  cones  are  lobed,  the  greatest  expan- 
sions being  opposite  the  mouths  of  the  larger  canyons,  thus 
giving  to  the  bases  of  the  desert  ranges  when  seen  from 
above  a  scalloped  outline.  Where  the  curving  margins  of 
the  alluvial  cones  are  drawn  in  toward  the  mountains  sur- 
rounded by  them,  they  meet  projecting  spurs  and  ridges 
from  which  there  is  but  little  drainage,  and  talus  slopes  fre- 
quently occur.  The  length  and  size  of  the  gorges  in  the 
mountains  may  thus  be  judged  by  the  extent  and  height  of 
the  alluvial  deposits  at  their  mouths. 

The  streams  reaching  the  alluvial  cones,  when  not  at  once 
absorbed,  build  up  their  sides  and  channels,  and  thus  be- 
come unstable.  The  waters  break  through  the  ridges  formed 
along  their  margins,  and  branch  or  bifurcate  in  various 
directions,  and  are  thus  led  from  time  to  time  over  all  por- 
tions of  the  surfaces  of  the  cones,  and  distribute  their  bur- 
dens evenly  upon  them.  This  tendency  of  the  supplying 
streams  to  bifurcate  and  send  off  distributaries  is  of  the 
same  nature,  as  will  be  noted  later,  as  occurs  on  deltas.  In 
fact,  the  upper  portion  or  cap  of  a  delta  built  by  a  high- 
grade  stream,  is  an  alluvial  cone,  having  all  of  the  charac- 
teristic features  of  the  example  under  discussion.  The 
abandoned  courses  of  distributaries  on  the  surfaces  of 
alluvial  cones  are  frequently  marked  by  parallel  ridges  with 


■'V: 


a 

1 


104 


RIVERi    OF  NORTH  AMERICA 


•ill     ;': 


a  stream  bed  between.  The  branching  of  the  streams  is 
also  frequently  well  mapped  in  this  manner  during  the  long 
intervals  between  ::torms,  when  no  water  reaches  the  alluvial 
deposits.  Sometimes  huge  boulders  occur  on  the  surfaces 
of  the  alluvial  cones  of  the  Far  West,  two  or  three  miles 
from  the  mouths  of  the  gorges  from  which  they  came,  and 
remain  as  records  of  the  violence  of  the  streams  during 
storms. 

Normally  the  surface  of  the  alluvial  cones,  in  the  region 
just  referred  to,  is  bare  of  all  vegetation  except  desert 
shrubs  and  grasses,  while  at  their  heads  and  in  the  mouths 
of  the  feeding  gorges  there  are  frequently  clumps  of  willows, 
alders,  and  other  less  familiar  shrubs.  Occasionally  the 
slight  perennial  drainage  from  the  mountains,  which  is  con- 
cealed by  the  debris  in  the  bottoms  of  the  gorges,  comes  to 
the  surface  at  the  head  of  an  alluvial  cone,  and  forms  a 
spring  towards  which  the  cattle  and  game  trails  in  the  valleys 
converge. 

During  the  rainy  season  each  year  the  gorges  in  the 
mountains  are  occupied  by  streams  which  bring  down  debris 
and  make  additions  to  the  alluvial  deposits,  but  in  summer 
all  contributions  cease,  except  during  occasional  heavy  rr  On- 
falls. The  growth  of  the  alluvial  cones  is  thus  markedly 
spasmodic.  When  the  heavy  rains,  usually  termed  cloud- 
bursts, occur,  ton  its  rush  down  the  mountain  gorges  and 
carry  with  them  vast  quantities  of  debris  which  had  been 
slowly  accumulating  in  their  channels,  possibly  for  scores  of 
years,  and  marked  additions  are  made  to  the  alluvial  cones 
in  the  valley  below.  Between  these  occasional  catastro- 
phes, disintegration  of  the  rocks  goes  on,  and  the  gorges 


STREAM  DEPOSITS 


105 


again  become  charged  with  rock  fragments.  During  the  in- 
tervals between  floods,  the  winter  streams  frequently  remove 
some  cr  the  mate/ial  previously  deposited,  and  a  notch  is 
cut  in  the  apex  of  the  alluvial  cone.  Many  alluvial  cones  in 
Utah,  Nevada,  and  adjacent  regions,  are  thus  notched  at 
their  summits,  so  that  in  ascending  them  in  order  to  gain 
the  gorge  above,  one  passes  through  a  trench  in  alluvium, 
possibly  a  hundred  feet  or  more  in  depth.  The  sides  of 
these  gorges  reveal  sections  of  the  alluvial  deposit,  and  show 
that  the  material  is  rudely  stratified.  The  strata  are  rot 
horizontal,  but  inclined  in  conformity  with  the  surface 
slopes  of  the  cones,  and  show  cross-bedding  and  many  other 
irregularities.  In  general,  the  material  composing  an  allu- 
vial cone  is  coarse  near  the  entrance  to  the  gorge  to  which 
it  leads,  and  fine  on  its  outskirts  in  the  valley,  but  even  on 
the  lower  borders  coarse  gravel  and  stones  may  occur. 
i  Within  the  gorges  cut  in  the  summit  portions  of  these 
conical  piles  there  are  frequently  terraces,  which  record 
various  stages  in  the  down-cutting  performed  by  the 
streams.  The  reason  why  a  small  stream  fed  by  winter 
rain  and  melting  snow  is  able  to  excavate  a  channel  in  the 
apex  of  an  alluvial  cone  that  has  been  built  up  during 
occasional  heavy  storms,  sterns  to  be  because  during  the 
normal  winter  flow  the  streams  are  less  heavily  charged  with 
debris  than  at  the  time  of  the  occasional  floods,  and  can 
expend  a  portion  of  their  energy  in  corrading.  The  light 
loads  of  the  normal  winter  streams  can,  »n  some  instances, 
be  accounted  for  by  the  fact  that  the  canyons  have  been 
cleared  of  available  debris  by  a  previous  great  storm. 
Another  condition  that  may  favour  the  cutting  of  the  chan- 


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RIVERS  OF  NORTH  AMERICA 


riel  referred  to,  is  that  during  times  of  slack  drainag-"  the 
waters  are  absorbed  by  the  alluvium,  and  the  silt  brought 
by  them  is  left:  in  the  interstices  in  the  coarser  debris,  and 
the  filling  the  interspaces  causes  the  waters  to  be  retained 
at  the  surface,  thus  allowing  them  to  corrade.  Yet  another 
change  in  conditions  which  leads  to  the  cutting  of  the  chan- 
nels referred  to,  and  one  which  occurs  also,  as  will  be  con- 
sidered later,  in  the  history  of  flood-plains,  is  a  result  of 
normal  stream  development.  The  removal  of  material  from 
a  mountain  gorge  when  corrasion  is  in  excess  of  weathering, 
causes  a  lowering  of  the  gradient  above  the  apex  of  the 
alluvial  cone  at  its  mouth,  and  necessitates  a  readjustment 
of  grade  in  the  lower  course  of  the  stream  where  deposition 
was  previously  in  progress;  this  is  accompHshed  by  e 
cutting  of  a  channel  through  the  apex  of  the  alluvial 
and  the  re-deposition  of  the  material  removed  lower  down 
on  its  surface.  In  such  an  instance  a  second  cone  is  built 
on  the  surface  of  the  first  one  formed,  with  its  apex  in  the 
notch  excavated  in  the  summit  of  the  earlier  accumulation. 
Many  alluvial  cones  are  for  this  reason  compound  structures. 
Still  greater  complexity  of  the  same  character  occurs  in  the 
flood-plain  deposits  of  large  rivers,  as  will  be  shown  later. 

The  fact  that  alluvial  cones  are  more  common  and  more 
conspicuous  in  arid  than  in  humid  regions  depends  on  a 
variety  of  circumstances.  One  of  the  controlling  conditions 
favouring  their  growth  is  an  abrupt  change  in  the  grade  of 
a  stream  bed.  In  humid  regions,  the  grade  of  streams  is 
more  quickly  adj-isted  than  when  the  climate  is  arid  and  the 
streams  intermittent.  In  humid  regions,  also,  the  streams 
are  more  constant,   and,   instead  of  terminating  in  desert 


STREAM  DEPOSITS 


107 


valleys,  flow  on  and  carry  their  loads  to  lakes  or  to  lower 
regions  instead  of  dropping  them  at  one  locality.  Then, 
too,  in  humid  regions,  there  are  usually  streams  in  the 
valleys  bordering  the  mountains,  to  which  the  torrents  are 
tributary.  The  valley  streams  tend  to  cut  away  and  remove 
the  alluvial  deposit  on  their  border,  and  thus  prevent  great 
accumulation  of  debris  at  the  mouths  of  lateral  gorges. 
Abundant  rain-fall  and  vegetation  also  have  an  important 
influence  on  the  deposits  that  may  be  formed.  The  rocks 
decay  more  rapidly,  and  sub-aerial  deposits  of  debris  are 
usually  more  quickly  removed,  in  humid  than  in  arid  cli- 
mates, even  when  the  mean  annual  temperature  is  the  same. 
Vegetation  not  only  masks  the  de  )sits  of  the  nature  here 
considered,  but  assists  in  various  ways  in  their  removal. 

The  principal  conditions  favouring  the  origin  and  growth 
of  alluvial  cones  are  high-grade  gorges  leading  to  adjacent 
valleys  in  which  there  are  no  streams,  and  climatic  condi- 
tions favourable  to  great  fluctuations  in  the  flow  of  the 
mountain  torrents  and  also  to  rock  disintegration  rather 
than  rock  decay. 

As  the  rocks  forming  a  mountain  in  an  arid  region  are 
shattered  and  disintegrated  by  changes  of  temperature  and 
other  similar  agencies,  the  loosened  material  is  washed  down 
the  gorges  and  deposited  in  alluvial  cones,  in  the  manner 
just  considered.  As  the  alluvial  cones  about  the  base  of  a 
mountain  gradually  increase,  their  apexes  progress  farther 
and  farther  up  the  gorge,  and  should  the  favourable  climatic 
conditions  prevail  sufficiently  long,  the  alluvial  deposits 
begun  on  opposite  sides  of  a  range  may  grow  until  they 
meet  on  the  divide  at  the  crest  of  the  mountain.     The  dis- 


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RIVERS  OF  NORTH  AMERICA 


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integration  of  the  still  exposed  peaks  and  ridges  will  con- 
tinue, and  not  only  will  the  main  valleys  become  choked 
with  rock  fragments,  but  each  lateral  gorge  will  also  contain 
an  extension  of  an  alluvial  cone.  In  time  the  exposed  pin- 
nacles and  ledg^  s  of  rock  will  become  covered  with  loosened 
fragments  and  the  ent're  range  assume  subdued  and  flowing 
outlines.  The  core  of  still  solid  rock  will  then  be  covered 
and  concealed  by  a  sheet  of  disintegrated  fragments,  which 
will  be  thickest  in  the  former  gorges.  There  are  mountains 
in  the  arid  regions  referred  to  above  which  have  become 
buried  in  this  manner  in  their  ow.i  debris. 

The  life  histories  of  alluvial  cones  are  long  and  remark- 
ably unvaried.  The  chief  episodes  in  their  lives  are  the 
sudden  and  severe  storms  which  increase  their  rate  of 
growth  in  a  striking  manner.  They  continue  to  increase  in 
all  of  their  dimensions  as  long  as  there  is  high  land  to  fur- 
nish debris,  unless  climatic  conditions  change  and  the 
streams  which  supply  them  become  perennial.  If  the 
climate  remains  arid,  however,  they  gradually  expand,  per- 
haps with  the  engrafting  of  secondary  or  parasitic  cones  on 
their  sides,  until  the  mountain  about  which  they  began  to 
form  is  buried,  and  then,  owing  to  decay  and  the  removal 
of  material  in  solution,  flatten  out  and  become  hills  with 
gracefully  flowing  outlines,  which  in  time  slowly  fade  away. 

The  reader  will  no  doubt  fancy  that  I  have  given  too 
much  space  to  the  consideration  of  this  special  phase  of 
stream  deposition,  and  one  which  is  of  minor  importance 
when  all  of  the  accumulations  made  by  streams  are  con- 
sidered, but  the  study  of  alluvial  cones  assists  in  an  import- 
ant way  in  understanding  the  nature  of  all  other  stream- 


Plate  VII. 


Fir..  A.     Valley  of  Big  Goose  River,  Wyoming. 
An  old,  alluvial-filled  valley  with  a  meandering  stream.    (Photograph  by  W.  H.  Jackson.) 


n. 
II  if. 


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Fio.  B.     New  River,  Tennessee. 
A  young  valley  excavated  in  a  peiieplain.    (Photograph  by  M.  R.  Campbell.) 


#.»• 

If 

»•{ 


STREAM  DEPOSITS 


109 


made  deposits.  The  upper  portions  of  deltas,  for  example, 
are  simply  alluvial  cones.  Even  the  flood-plains  of  rivers 
are  imperfectly  developed,  or,  perhaps  more  properly,  com- 
posite, alluvial  cones  with  greatly  extended  sides  and  low 
surface  slopes.  The  alluvial  cones  of  arid  regions  and  the 
alluvial  caps  of  deltas  are  usually  allowed  to  expand  sym- 
metrically, but  flood-plains,  as  will  be  shown  later,  are  con- 
fined by  the  sides  of  the  valleys  in  which  they  are  formed, 
and  may  be  considered  as  long,  narrow,  longitudinal  sections 
of  deposits  of  a  similar  origin. 

TALUS    SLOPES 

Alluvial  cones  frequently  unite  with  or  merge  into  the  ac- 
cumulations of  debris  which  form  at  the  bases  of  cliffs  and 
are  supplied  directly  by  rock  fragments  falling  from  the 
precipices  above  them.  These  talus  slopes,  or  screes,  as 
they  are  sometimes  termed,  are  composed  of  angular  frag- 
ments of  rocks,  although  frequently  rounded  by  weathering, 
of  all  sizes  up  to  many  cubic  feet,  and  have  surface  slopes 
frequently  as  great  as  thirty  or  thirty-five  degrees.  Unless 
cut  away  at  the  base,  as  by  a  stream,  the  surface  slopes  re- 
present the  angle  of  repose  of  the  material  of  which  the 
apron-like  piles  are  composed. 

Talus  slopes  are  not  stream  deposits,  but  are  frequently 
associated  with  alluvial  cones,  and  may  be  mistaken  for 
them.  They  differ  from  alluvial  cones,  however,  in  their 
mode  of  origin,  and  in  the  fact  that  they  are  not  composed 
of  water-worn  material,  unless  the  cliffs  above  them  contain 
conglomerate,  and  usually  have  much  higher  surface  slopes. 


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RIVERS  OF  NORTH  AMERICA 


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The  spaces  between  the  large  blocks  are  not  usually  filled, 
and  the  material  of  which  they  are  composed  is  not  stratified. 
These  differences,  and  the  fact  that  talus  slopes  occur  at 
the  base  of  precipices,  while  alluvial  cones  are  built  at 
the  mouth  of  gorges,  enable  one  to  readily  discriminate 
between  them. 

FLOOD-PLAINS 

It  is  well  known  that  streams,  whether  riiL  or  rivers, 
seldom  follow  a  straight  course  for  any  considerable  dis- 
tance, unless  held  in  a  definite  channel  for  a  time  by  walls 
of  solid  rock.  The  path  of  a  stream  is  a  series  of  curves. 
Owing  to  the  deflection  of  the  thread  of  swiftest  current, 
the  banks  on  the  concave  or  outer  curves  are  eaten  away, 
while  those  bordering  the  convex  or  inner  curves  are  added 
to.  As  has  been  described  in  considering  the  meandering 
of  streams,  any  inequality  in  their  bottoms  or  banks  may 
lead  to  a  deflection  of  the  current.  If  we  imagine  a  stream 
flowing  through  a  broad  valley,  even  if  of  constant  volume, 
this  process  of  meandering  from  side  to  side  in  a  tortuous 
course  will  go  on.  As  the  stream  bed  is  shifted,  deposits 
are  laid  down  on  the  border  of  the  convex  curves,  and  new 
land  formed.  The  first  deposits  made  are  of  coarse  material, 
usually  well-rounded  gravel  or  stones,  next  above  this  layer 
finer  material  is  deposited,  and  last  of  all,  fine  silt.  If  the 
stream  migrates  from  one  side  of  its  valley  to  the  other,  one 
bank  is  progressively  cut  away  and  the  material  thus  re- 
moved in  part  re-deposited  lower  down  on  the  other  side, 
and  at  the  same  time  assorted.  In  this  manner  a  plain  is 
formed,    having   an  even  surface   of  fine   rich    soil   which 


STREAM  DEPOSITS 


III 


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IS 


changes  towards  its  base  into  coarser  and  coarser  material. 
When  the  meandering  of  the  stream  is  reversed  and  it 
swings  back  again  across  the  v'alley,  the  material  it  pre- 
viously deposited  is  again  cut  away  and  again  carried  lower 
down  and  re-deposited  on  the  slack-water  sides  of  lower 
curves,  and  is  again  assorted  during  the  process.  In  this 
manner  the  material  flooring  a  river  valley  is  worked  over 
and  over  many  times. 

The  manner  in  which  streams  cut  away  the  land  on  the 
outer  sides  of  stream-curves  so  as  to  produce  steep  banks, 
and  deposit  material  on  their  inner  or  convex  sides  so  as  to 
form  gentle  slopes,  is  illustrated  by  the  view  of  Indian 
Creek,  California,  presented  on  Plate  VI.  Other  character- 
istic examples  of  meandering  streams  are  shown  on  Plate  III. 

In  the  case  of  a  stream  without  marked  variations  in  vol- 
ume, flood-plain  building  goes  on  slowly,  or  rather,  perhaps, 
an  approximate  equilibrium  is  first  reached  and  then  the 
change  is  slow,  but  is  probably  always  in  progress,  although 
varying  in  different  instances  according  as  the  stream  is 
clear  or  more  or  less  charged  with  sediment.  In  nature  the 
process  just  outlined  is  almost  always  assisted  by  variation 
in  the  volume  of  a  stream.  During  periods  of  decreased 
rain-fall,  as,  for  example,  in  the  summer  season  in  most 
regions,  the  streams  are  low  and  confined  to  their  immediate 
channels;  when  storms  come,  however,  or  the  snow  melts, 
the  streams  are  swollen  and  more  than  fill  their  summer 
channels.  It  is  during  floods  that  the  greater  amount  of 
corrasion  on  the  co.icave  curves  and  of  deposition  on  the 
convex  curves  takes  place. 

There  are  certain  limitations  to  the  extent  a  stream  may 


«■««» 


112 


HI  VERS  OF  NORTH  AMERICA 


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meander  to  the  right  and  left  of  its  general  course,  inherent 
in  the  process  by  which  curves  are  formed,  which  deter- 
mine the  width  of  the  belt  of  country  it  works  over.  In 
studying  these  limitations,  however,  a  distinction  should  be 
borne  in  mind  between  the  comparatively  small  curves 
originating  in  the  manner  just  described,  and  the  much 
greater  sweeps,  made  up  of  many  small  curves,  which  I 
have  termed  migrations,  as,  for  example,  the  great  swings 
of  the  Mississippi  from  side  to  side  of  its  broad  alluvial 
valley  below  the  mouth  of  the  Ohio.  The  checks  men- 
tioned below  on  the  tendency  of  a  stream  to  meander  in 
small,  sharp  curves  do  not  seem  to  be  operative,  or  at  least 
do  not  act  in  the  same  manner,  in  the  case  of  the  broad 
migrations. 

When  a  stream  forms  an  oxbow  curve,  it  is  plain  that  its 
course  is  lengthened,  and  that  aggrading  must  take  place  in 
order  to  allow  the  waters  to  carry  their  burden  over  the  ex- 
tended course.  The  bed  of  the  stream  at  the  beginning  of 
a  curve  will  thus  be  raised  higher  and  higher,  as  the  curve 
increases  in  length.  During  times  of  flood  when  the  stream 
overspreads  its  banks,  the  waters  cross  the  narrow  neck  of 
the  oxbow  curve  and  begin  to  excavate  a  channel.  Once  a 
start  is  made  the  entire  river  soon  takes  the  shorter  and 
steeper  course,  and  the  curve  is  cut  off.  The  entrance 
and  exit  of  the  abandoned  curve  are  soon  silted  up,  on  ac- 
count of  the  deposition  of  debris  in  the  slack  water  in  the 
embayments  on  the  side  of  the  straightened  stream,  and 
an  "  oxbow  lake  "  is  the  result. 

As  cited  by  C.  R.  Keyes,'  the  difference  in  elevation  on 

^Missouri  Gfological  Survey,  vol.  x.,  ji.  97,  1896. 


STREAM  DEPOSITS 


i'3 


the  two  sides  of  the  neck  of  an  oxbow  curve  is  frequently 
sufficient  for  a  small  ravine  to  be  cut  backward  from  the 
lower  side  so  as  to  make  a  slight  rent  in  the  narrow  strip  of 
land,  thus  directing  and  facilitating  the  work  of  the  over- 
flowing waters  when  a  flood  occurs. 

There  is  thought  to  be  a  definite  limit  established  in  this 
manner  on  the  extent  to  which  a  stream  may  meander,  but 
on  account  of  the  numerous  varying  conditions  entering 
into  the  problem, — such  as  the  volume  of  a  stream,  its 
velocity,  its  fluctuations,  the  influence  of  vegetation,  the 
nature  of  its  banks,  and  in  many  regions  the  influence  of 
ice, — it  has  not  been  found  possible  to 'determine  how 
widely  any  given  stream  may  meander. 

The  broad  migrations  referred  to  above  depend  on  con- 
ditions still  more  complex  than  those  limiting  a  stream's 
meanderings,  and  as  they  have  not  been  carefully  studied, 
it  is  perhaps  best  not  to  attempt  to  discuss  them  at  this 
time. 

During  floods,  also,  a  stream  frequently  inundates  the 
plains  formed  during  previous  meanderings.  The  region 
thus  submerged  is  for  this  reason  termed  di  flood-plain.  The 
waters  on  spreading  from  the  main  channel  lose  velocity, 
and  the  silt  they  hold  in  suspension  is,  to  a  great  extent, 
precipitated.  A  layer  of  fine  material  is  thus  spread  over 
the  plain  and  raises  it  by  the  addition  of  a  layer  of  rich  soil. 

The  debris  brought  from  the  upper  portion  of  a  stream 
course,  where,  on  account  of  the  high  grade,  corrasion  is  in 
progress,  is  thus  dropped*in  the  valley  and  plain  tracts,  and 
not  only  raises  the  bed  of  the  stream  but  is  built  into  flood- 
plains. 


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114 


RIVERS  OF  NORTH  AMERICA 


Flood-plains  of  the  character  just  described  border  prac- 
tically all  the  streams  of  North  America,  below  the  region 
of  active  corrasion,  and  furnish  some  of  the  richest  and 
most  easily  cultivated  lands  the  continent  possesses.  The 
flood-plains  of  the  Mississippi,  for  example,  are  from  ten  to 
fifty  or  sixty  miles  broad,  and  in  the  neighbourhood  of  fifty 
thousand  square  miles  in  area.  In  the  settlement  of  a  new 
country  the  flood-plains  are  usually  the  first  areas  occupied, 
not  only  on  account  of  their  fertility,  but  because  of  their 
ready  access.  The  main  obstacle  met  with  by  early  settlers 
is  usually  the  heavy  growth  of  timber  that  has  to  be  cleared 
away  in  order  to  admit  of  the  cultivation  of  the  soil. 
Another  difficulty  arises  from  the  fact  that  the  flood-plains 
are  subject  to  inundation  at  times  of  high  water,  as  is  well 
known  in  the  case  of  the  lower  Mississippi.  After  the  floods 
have  subsided,  the  moist  lowlands  are  in  many  instances 
unhealthy  and  malarial  fevers  are  apt  to  be  prevalent. 

It  is  of  interest  to  note  that  flood-plains  begin  to  form 
where  a  stream  loses  grade  and  is  no  longer  able  to  trans- 
port the  debris  brought  down  its  higher-grade  upper  course. 
Usually  they  first  appear  as  one  descends  a  river,  at  the 
lower  end  of  its  mountain  tract,  and  extend  from  that 
locality  to  the  sea.  Their  similarity  to  alluvial  cones  is 
thus  made  apparent.  An  alluvial  cone  is  in  fact  a  flood- 
plain,  formed  under  special  conditions.  The  surface  slopes 
of  alluvial  cones  are  high  in  comparison  with  what  may  be 
termed  normal  flood-plains,  because  the  gorges  of  the  short 
streams  supplying  them  are  of  higher  grade  than  the  larger 
streams  which  build  long  flood-plains.  The  purpose,  as  we 
may  say,  of  the  alluvial  cone  is  to  grade  up  the  lower  course 


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STREAM  DEPOSITS 


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of  a  stream  to  conform  with  its  slope  in  its  upper  portion, 
and  thus  furnish  an  even  grade  down  which  the  stream  may 
carry  its  burdens.  The  sam.e  is  true  of  flood-plains;  the 
river  has  more  material  delivered  to  it  at  the  head  of  its 
valley  tract  than  it  can  continue  to  carry  with  the  current 
necessitated  by  the  lower  slope  farther  downstream,  and  at 
once  begins  to  raise  its  grade.  The  aggrading  progresses 
all  the  way  from  the  locality  where  the  stream  begins  to  be 
overloaded,  throughout  its  lower  course  to  the  sea,  unless 
rapids  break  its  course.  When  rapids  occur,  each  separate 
portion  of  the  stream  behaves  like  an  independent  river, 
and  each  section  deepens  its  channel  or  fills  it  in,  as  the 
case  may  be. 

In  an  alluvial  cone  the  greatest  depth  of  the  deposit  is 
near  its  head,  and  probably  at  the  locality  where  deposition 
first  began.  In  a  flood-plain,  in  general,  the  greatc  t  depth 
of  the  deposit  is  near  the  lower  end  of  the  mountain  tract 
and  decreases  all  the  way  to  the  sea.  The  locality  of 
greatest  depth  of  filling,  however,  will  migrate  up  stream  as 
the  flood-plain  increases  in  length.  Unless  the  stream  is 
building  a  delta  and  thus  lengthening  its  course,  the  flood- 
plain  at  its  mouth  cannot  be  raised.  Hence  deposition  on 
a  flood-plair;  at  sea-level  is  practically  nil. 

If  the  changt.s  experienced  by  a  stream  from  youth  to 
old  age  are  followed,  it  will  be  found  that  its  valley,  plains, 
and  mountain  tracts  are  mutually  extended  up  stream.  The 
plains  tract  especially  increases  in  length,  while  the  mountain 
tract  becomes  shorter  and  shorter  unless  it  is  so  situated 
that  it  can  extend  its  branches  headward.  With  the 
lengthening  of  the  portion  of  the  stream  characterised  by 


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RIVERS  OF  NORTH  AMERICA 


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low  gradients — that  is,  of  the  plains  and  valley  tracts, — there 
is  an  up-stream  migration  of  the  locality  where  flood-plains 
begin.  These  normal  phases  of  stream  development,  how- 
ever, will  be  more  readily  understood  after  the  adjustment 
of  streams  to  the  structure  of  the  rocks  corraded  by  them 
has  been  considered.  The  fact  that  a  stream  under  certain 
conditions,  already  noted,  cuts  a  channel  through  the  upper 
portion  of  an  alluvial  cone  and  forms  terraces  in  the  gorge 
thus  produced,  will  assist  us  later  in  understanding  the  man- 
ner in  which  streams  may  cut  channels  through  their  flood- 
plains,  and  form  terraces  on  the  side  of  such  channels. 

':7v'  'J:.-:.,".-v    ■  NATURAL  LEVEES  -     ''    ; ' 

On  visiting  a  stream  that  is  subject  to  floods,  and  carries 
a  heavy  load  of  silt  especially  during  high-water  stages,  one 
will  usually  find  that  its  immediate  banks  are  higher  than 
the  general  level  of  the  surface  of  the  adjacent  flood-plain, 
and  form  ridges.  In  the  case  of  creeks  and  small  rivers, 
the  ridges  may  be  a  hundred  feet  across,  and  their  crests 
five  to  six  feet  or  more  above  the  surface  of  the  flood-plain 
or  back  country,  as  it  is  frequently  termed.  In  many  in- 
stances, too,  the  back  country  between  the  river  and  the 
bluffs  bordering  the  valley,  is  imperfectly  drained  and  in  a 
swampy  condition.  The  region  referred  to  is  frequently 
densely  covered  with  alders  and  other  swamp-loving  plants, 
and  may  support  great  cottonwoods  and  other  forest 
growths.  Through  the  forests  one  frequently  finds  open, 
park-like  passages,  clear  of  vegetation,  which  mark  second- 
ary channels,   occupied   by   branches  of  the  main  stream 


STREAM  DEPOSITS 


117 


during  high-water  stages.  The  embankments  referred  to 
on  each  border  of  a  stream,  where  it  flows  through  a  flood- 
plain,  are  built  up  by  the  river.  As  the  waters  rise  during 
floods,  and  spread  beyond  the  border  of  the  low-water 
channel,  the  current  is  checked,  and  they  drop  all  but  the 
very  finest  of  the  material  held  in  suspension.  It  thus  hap- 
pens that  the  thickest  deposits  are  made  on  the  immediate 
border  of  the  channel,  and  ridges  which  follow  all  the  mean- 
derings  of  the  stream  are  built  up.  These  embankments 
resemble  the  levees  built  by  man  in  attempting  to  confine  a 
stream  to  its  immediate  channel,  and  are  hence  known  as 
natural  levees.    .  / 

When  the  waters  of  a  rising  river  pass  the  natural  levees, 
they  are  still  charged  with  silt,  much  of  which  is  precipitated 
on  the  inundated  country  and  adds  to  the  layer  of  fine  ma- 
terial found  on  the  surface  of  all  low-grade  flood-plains.  It 
is  to  the  addition  of  this  thin  layer  of  exceedingly  fine 
mud,  mingled  in  many  instances  with  organic  matter,  derived 
from  the  working  away  v^f  soil  farther  upstream,  that  the 
wonderful  fertility  of  flood-plains  is  largely  due.  The  decay 
of  the  rank  vegetation  on  many  flood-plains,  especially 
during  the  time  previous  to  the  clearing  of  the  land  for 
cultivation,  also  adds  to  the  fertility  of  "  river  bottoms,"  as 
the  flood-plains  are  frequently  termed. 

While  natural  levees  are  a  protection  to  the  back  country 
during  moderate  floods,  serving  as  they  do  to  retain  the 
waters  in  a  definite  channel,  yet,  as  has  been  found,  espe- 
cially in  the  case  of  the  Mississippi,  they  are  also  a  source  of 
danger.  During  unusually  high  floods,  the  rivers  break  the 
embankment  confining   them,  and  branching  streams  are 


ii8 


RIVERS  OF  NORTH  AMERICA 


I 


Ji 
s 

»  '..■, 

I'' 

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tl- 


formed,  which  carry  destruction  in  their  paths  and  sweep 
away  the  fields  and  whatever  else  obstructs  their  flow. 

The  typical  cross-profile  of  a  river  valley  in  which  levees 
occur,  as  has  been  shown  by  Hicks,'  does  not  present  a 
straight  line  from  the  stream's  channel  to  the  bordering 
bluffs,  but  a  double  curve.  If  no  terraces  are  present  on 
the  sides  of  the  valley,  each  half  presents  a  weather-curve 
at  the  top  of  the  bluff,  and  two  other  curves  of  opposite 
character,  one  concave  due  to  erosion,  and  the  other  convex 
due  to  deposition.  At  the  intersection  of  the  water-curves 
there  is  usually  a  swamp.  These  features,  somewhat  ex- 
aggerated in  the  vertical  scale,  are  shown  in  the  following 
diagram. 


Fig.  4.     Cross-Profile  of  a  Valley  Occupied  by  a  Constructive  River.     After 

L    '-.  Hicks.) 

a  b.  Weather-Curve  at  Crest  of  Bluffs;  b  c.  Water-Curve  of  Corrasion ;  c  d.  Swamp;  de. 
,  .  Water-Curve  of  Deposition  ;  J.  Stream  Channel. 

The  water  which  covers  a  flood-plain  between  the  levees 
and  the  uplands  bordering  a  valley  during  flood,  finds  its 
way  back  to  the  river  again  at  some  locality  lower  down- 
stream, usually  by  entering  a  tributary.  In  the  lower  Mis- 
sissippi, much  of  the  water  that  escapes  in  this  manner 
from  the  main  channel  is  carried  to  the  Gulf  of  Mexico  by 
distributaries  from  the  main  stream. 

Where  natural  levees  are  built,  the  bed  of  a  stream  is 

'  L.  E.  Hicks,  "  Some  Elements  of  Land  Sculpture,"  in  Bulletin  of  the  Geo- 
logUal  Society  of  America,  vol.  iv.,  p.  143,  1893. 


I 


I20 


RIVERS  OF  NORTH  AMERICA 


usually  raised  by  deposits  made  in  its  bottom.  In  this  way- 
it  happens  that  a  stream  sometimes  flows  in  a  trench  on  the 
top  of  a  ridge  more  elevated  than  the  back  country,  and  is 
thus  in  an  unstable  position,  and  liable  to  shift  its  channel 
when  l^reaks  in  the  levees  occur.  This  is  one  method  by 
which  streams:  are  led  over  all  portions  of  the  valleys  they 
occupy. 

Illustrations  of  the  manner  in  which  a  river  subject  to 
floods  breaks  through  its  levees  from  time  to  time  and  in- 
undates its  flood-plain  are  furnished  nearly  every  year  by 
the  Mississippi,  especially  below  the  mouth  of  the  Ohio.  In 
the  case  of  this  river,  however,  the  extent  of  the  inundation 
is  increased  in  many  instances  by  the  attempts  that  have 
been  made  to  confine  it  to  its  main  channel  by  adding  to 
the  height  of  the  natural  levees.  Under  the  artificial  con- 
ditions imposed  on  the  river,  when  breaks,  or  crevasses,  as 
they  are  termed,  in  the  levees  do  occur,  plantations  are 
ruined,  buildings  swept  away,  and  in  some  instances  the 
moUusks,  fishes,  and  other  animals  «n  the  bays  and  sounds 
bordering  the  deltas  are  destroyed  by  the  vast  quantities  of 
fresh  water,  charged  with  mud,  poured  into  them. 

The  extent  of  the  inundation  in  the  lower  Mississippi,  in 
the  spring  of  1890,  is  shown  on  the  accompanying  map. 
The  shaded  portion  of  the  map  indicates  the  regions  that 
were  covered  by  the  overflowing  waters,  while  the  unshaded 
portion  reveals  the  distribution  of  the  land  too  high  to  be 
reached  by  the  flood.  It  is  instructive  to  note  that  the  un- 
submerged  land  lies  close  along  the  borders  of  the  streams, 
and  indicates  the  position  of  their  embankments.  At  the 
time  of  the  inundations  referred  to,  the  water  broke  through 


STREAM  DEPOSITS 


121 


the  left  embankment  of  the  river  at  what  is  known  as  the 
Nita  crevasse,  about  twenty  miles  above  New  Orleans,  and 
formed  a  current  of  fifteen  miles  an  hour,  which  carried  de- 
struction in  its  path.  The  escaping  water,  joining  that 
from  another  break  known  as  the  Martinez  crevasse,  ap- 
proximately twelve  miles  in  a  straight  line  higher  up  the 
stream,  flowed  eastward  and  caused  Lake  Maurepas  and 
Lake  Pontchartrain  (six  hundred  square  miles  in  area)  to 
overspread  their  banks.  The  water  flowed  over  the  country 
eastward  to  Lake  Borgne  and  entered  Mobile  Bay,  with 
such  volume  as  to  cause  a  current  eastward  and  destroy  for 
a  time  the  important  oyster  and  fish  industries  of  that  arm 
of  the  sea.'  v       ' 

The  study  of  flood-plains  and  the  manner  in  which 
streams  divide  near  their  irouths  when  aggrading,  or  delta- 
bnilding,  is  in  progress,  illustrates  a  natural  method  by 
which  inundations  of  the  country  bordering  the  lower  course 
of  an  alluvial  river  are  lessened  or  prevented.  When  a  dis- 
tributary leaves  a  trunk  stream,  the  volume  of  water  below 
the  place  of  escape  is  lessened,  and  the  tendency  to  break 
across  ♦he  levees  decreased.  The  practice  of  building 
artificial  levees,  so  frequently  resorted  to  in  the  vain  at- 
tempt to  control  large  rivers  subject  to  high-water  stages, 
checks  this  natural  tendency.  If,  for  example,  instead  of 
attempting  to  confine  the  Mississippi  to  a  single  channel  in 
its  lower  course,  the  natural  distributaries  could  be  cleared 

'  L.  C  Johnson,  "The  Nita  Crevasse,"  in  HulUtin  of  the  Gfological  Socitty 
of  America,  vol.  ii.,  pp.  20-25,  ifiQi. 

The  areas  overflowed  by  the  Mississippi  in  1897  are  shown  on  an  excellent 
map  forming  I'late  II.  of  Park  Morrill's  report  on  the  "  Floods  of  the  Mississippi 
Kiver."    Tulilished  by  the  Weather  Bureau,  Woshington,  l8q7. 


tr 


t 


4 


j  I 


IM 


t  'H 


122 


RIVERS  OF  NORTH  AMERICA 


of  driftwood  or  other  obstructions  and  enlarged,  the  excess 
of  water  during  floods  might  be  drawn  off,  thus  lessening 
the  danger  to  the  levees  farther  seaward. 

The  influence  of  natural  levees  on  the  geography  of  a 
valley  where  aggrading  is  in  progress,  receives  additional 
illustration  from  the  manner  in  which  tributaries  of  the  main 
stream  are  deflected  from  what  would  otherwise  be  their 
natural  courses.  The  building  up  of  the  borders  of  the 
main  stream  necessitates  important  changes  in  its  tribu- 
taries. If  the  main  stream  increases  the  height  of  its  levees 
at  a  greater  rate  than  its  tributaries  can  aggrade  their  chan- 
nels, evidently  their  waters  must  either  rise  so  as  to  discharge 
across  the  dam  thus  formed,  or  be  turned  aside  and  flow  more 
or  less  nearly  parallel  with  the  main  stream  until  conditions 
favour  a  junction  with  it.  The  most  common  cause  for  such 
a  union  lies  in  the  fact  that  each  stream  is  meandering,  and 
sooner  or  later  one  or  the  other  will  cut  into  the  embank- 
ment built  by  its  neighbour.  The  origin  of  lakes  along  Red 
River,  Louisiana,  owing  to  the  raising  of  the  levees  on  its 
margins  faster  than  its  tributary  streams  are  able  to  aggrade 
their  channels,  has  been  described  by  Davis.'  An  examina- 
tion  of  a  good  map  *  of  the  lower  Mississippi  will  show  that 
its  tributaries  below  the  mouth  of  the  Ohio  frequently  ex- 
hibit an  abrupt  change  in  direction  after  entering  the  broad 
valley  of  the  main  rivfrr,  and  flow  parallel  with  it  for  many 
miles  before  being  able  to  effect  a  junction.     The  various 

'  W.  M.  Davis.  Scienct,  vol.  x.,  pp.  142,  143,  1887. 

*  An  excellent  map  wf  the  lower  Mississippi  valley  in  eight  sheets,  scale  five 
miles  to  an  inch,  has  been  published  by  the  U.  S.  Mississippi  River  Commis- 
sion, price  forty  cents.  This  can  be  obtained  by  applying  to  the  Secretary, 
Mississippi  Kiver  Commission,  St.  Louis,  Mo. 


STREAM  DEPOSITS 


123 


branches  of  the  Yazoo,  for  example,  after  entering  the 
valley  of  the  Mississippi,  flow  nearly  parallel  with  that  river 
for  about  two  hundred  miles,  but  join  it  at  Vicksburg, 
where  the  main  river  curves  eastward  and  washes  the  base 
of  the  bordering  bluffs,  leaving  no  room  for  a  secondary 
and  parallel  stream. 

DELTAS ' 

When  streams  deliver  their  loads  of  detritus  to  bodies  of 
still  water,  either  lakes  or  the  ocean,  deposition  takes  place 
in  much  the  same  way  as  when  alluvial  cones  are  formed, 
but  the  structure  and  shape  of  the  deposit  are  influenced  in 
an  important  manner  by  the  presence  of  the  still  water  into 
which  the  stream  discharges.  These  deposits,  when  seen 
from  above,  as  represented  on  a  map,  have  the  shape  of  an 
open  fan.  The  curved  margin  of  the  fan  faces  the  open 
water,  and  the  sharp  apex,  or  handle,  is  situated  at  the 
mouth  of  the  feeding  stream  or  well  up  its  valley.  The 
deposits  laid  down  in  the  Mediterranean  by  the  Nile  have 
the  characteristic  form  r  ferred  to,  except  that  the  fan  is  not 
fully  open ,  that  is,  the  outline  of  the  delta  is  a  triangle  and 
resembles  tl  Greek  letter  A,  as  was  long  since  noted ; 
hence  the  gei  ric  name  still  in  use.  All  deltas  do  not  ex- 
hibit the  characteristic  form  of  the  type  example,  however, 
but  may  be  markedly  semicircular  on  their  outer  margin,  or 
variously  lobed  or  indented,  as  is  shown  by  the  compound 

"  The  account  of  deltas  here  presented  is  essentially  the  same  as  may  he  found 
in  the  author's  book  entitled  Lakes  of  STorth  America,  one  of  tiie  series  of 
reading- lessors  to  which  the  present  volume  belongs,  but  this  repetition  is 
thought  de!>irable  in  order  to  make  each  book  as  nearly  complete  in  itself  as 
practicable.  ,,,  ,  • 


4 


ii 


<r 


124 


RIVERS  OF  NORTH  AMERICA 


delta  of  the  Mississippi.  An  inversion  of  the  A  form  in  the 
case  of  certain  small  deltas  in  Seneca  Lake  has  been  de- 
scribed by  the  writer.'  i     -  ;' 

The  conditions  governing  the  formation  of  deltas  are 
mainly  that  a  stream  shall  bring  detritus  to  a  body  of  water 
which  is  unaffected  by  strong  currents.  If  currents  exist  in 
the  receiving  water-body  where  the  stream  enters,  capable 
of  bearing  away  the  debris  delivered  to  it,  the  plan  of  the 
delta  will  be  modified,  and  if  the  currents  are  sufficiently 
strong,  all  of  the  material  is  carried  away  and  deposited 
over  the  bottom  or  built  into  bars  and  embankments  of 
various  forms  along  the  margin  of  the  lake  or  ocean.  It  is 
sometimes  stated  that  deltas  are  not  formed  in  water  bodies 
affected  by  a  tide,  but  this  is  an  indirect  explanation.  To 
be  sure,  deltas  are  seldom  found  along  the  shores  of  the 
ocean  where  the  rise  and  fall  of  the  tides  are  well  marked,  but 
this  is  for  the  reason  that  the  tides  usually  cause  currents 
which  bear  away  the  debris  brought  by  streams  as  fast  as 
delivered.  Water  bodies  sufficiently  quiet  to  favour  the 
growth  of  deltas  are  not  necessarily  tideless.  Currents  in 
water  bodies  are  also  produced  by  the  wind,  and  may  in- 
fluence delta-building  in  a  similar  way  to  tidal  currents. 
For  these  reasons,  sheltered  bays  and  estuaries,  where  the 
fluctuations  of  level  due  to  winds  and  tides  arc  not  excess- 
ive, are  among  the  most  iavourable  places  for  the  growth  of 
deltas. 

In  the  study  of  deltas,  it  is  convenient  to  divide  them 
into  two  classes,  namely,  those  made  by  high-grade  and 
consequently  rapid  streams,  and  those  made  by  low-grade 

•  I.  C.  Russell,  Lakes  of  North  America,  pp.  48-51.     Ginn  &  Co.,  1895. 


STREAM  DEPOSITS 


12$ 


and  therefore  comparatively  sluggish  streams.  These  dis- 
tinctions apply  of  course  only  to  the  lower  courses  of  the 
delta-making  streams.  Between  the  two  types  referred  to, 
a  complete  gradation  may  be  found. 

Deltas  of  High-Grade  Streams. — Typical  examples  of 
-deltas  of  this  class  occur  in  Utah  about  the  borders  of 
ancient  Lake  Bonneville,'  and  have  had  gorges  cut  through 
them  since  the  lake  surface  was  lowered.  A  radial  section 
of  a  delta  of  this  type,  such  as  would  be  exposed  in  the 
walls  of  a  trench  cut  through  it  from  apex  to  outer  margin, 
is  shown  in  Fig.  6,  page  126,  which  will  assist  the  reader 
in  understanding  the  leading  characteristics  of  such  deposits. 
A  swift  stream  is  able  to  bring  to  the  still  waters  of  a  lake, 
or  of  the  sea,  a  heterogeneous  load  of  detritus.  On  enter- 
ing the  receiving  water-body,  this  detritus  is  more  or  less 
perfectly  assorted.  The  coarser  or  heavier  portions,  namely, 
the  boulders,  gravel,  and  coarse  sand,  fall  to  the  bottom, 
while  the  lighter  and  finer  particles,  that  is,  the  fine  sand 
and  silt,  are  carried  farther.  The  very  fine  silt  is  floated 
away  from  the  mouth  of  the  feeding  stream,  and  slowly 
settling  to  the  bottom  forms  an  addition  to  the  sheet  of 
mud  which  is  being  laid  down  in  nearly  every  water-body. 

The  coarse  material  referred  to,  dropped  at  the  mouth  of 
the  stream  so  as  to  form  a  delta,  makes  an  addition  to  the 
land  and  the  stream  channel  is  lengthened.  The  outer 
border  of  this  deposit  is  steep  for  the  reason  that  the  debris 
as  it  is  dropped  tc  ids  to  form  a  pile  much  the  same  as  when 
similar  material  is  heaped  up  on  the  land,  the  supply  being 

'  G.  K.  (lilbcrt.  "  Lake  Bonneville,"  U.  S,  Gtological  Survey,  Monographs , 
vol.  i.,  pp.  65-70,  1890. 


Ifll 


A: 


126 


RIVERS  OF  NORTH  AMERICA 


from  the  top.  The  angle  of  repose,  that  is,  the  surface 
slope  of  material  deposited  under  water,  is  steeper  than 
would  be  assumed  by  the  same  material  accumulated  in  a 
similar  manner  on  land,  and  some  assorting  takes  place. 
Additional  material  brought  by  the  stream  is  carried  to  the 
top  of  the  steep  submerged  slope  referred  to,  and  rolls  down 
it  so  as  to  form  an  addition  to  the  inclined  layers.  As  a  delta 
grows  in  all  directions  from  the  mouth  of  the  feeding 
stream  in  which  the  water  has  freedom  to  flow,  and  invades 
deeper  and  deeper  portions  of  the  receiving  water-body, 
the  accumulation  of  inclined  layers  becomes  broader  and 
broader  at  the  same  time  that  its  outer  slope  or  under-water 
escarpment  increases  in  height.  The  angle  which  the 
escarpment  makes  with  a  horizontal  plane  varies  with  the 
size  and  shape  of  the  material  composing  it,  but  in  most 
instances  is  in  the  neighbourhood  of  thirty-five  degrees. 


^^-  ■ -V-J 


^         Fig.  6.     Radial  Section  of  a  Delta  Built  by  a  High-Grade  Stream. 

The  inclined  layers,  as  indicated  in  Fig.  6,  terminate 
upward  in  a  horizontal  plane,  which  coincides  with  the  sur- 
face of  the  receiving  water-body.  As  is  indicated  in  the 
diagram,  a  delta  built  by  a  high-grade  stream  has  three 
well-defined  portions;  the  middle  member  of  the  series 
being  the  debris  laid  down  in  inclined  layers,  in  the  man- 
ner just  described. 

The  fine  material  delivered  by  the  feeding  stream  is  car- 
ried beyond  the  outer  margin  of  the  system  of  inclined  beds 


STREAM  DEPOSITS 


127 


forming  the  medial  member  of  the  delta,  and  subsides  to 
the  bottom.  This  material  is  also  assorted.  The  coarser 
and  heavier  portions  reach  the  bottom  at  the  ir  mediate 
base  of  the  escarpment  formed  by  the  inclined  layers,  while 
the  finer  portions  are  carried  farther  before  coming  to  rest. 
In  this  manner  a  conical  deposit  of  fine  material  with  a  low 
surface  slope  is  made  about  the  base  of  the  accumulation  , 
of  inclined  layers.  As  the  delta  grows,  the  medial  member,  'i 
that  is,  the  system  of  inclined  beds  of  coarse  material, 
advances  over  the  finer  deposits  accumulated  about  its 
base,  and  in  many  instances  causes  them  to  be  pressed  out- 
ward and  variously  disturbed.  The  layers  of  fine  material 
are  frequently  folded  or  broken  by  the  weight  of  the  body 
of  coarse  debris  as  it  advances  upon  them.  : 

The  third  or  superior  portion  of  the  delta,  or  i\iQ  delta 
cap,  as  it  may  be  called,  is  an  alluvial  cone  which  is  built 
on  the  plane  formed  by  the  truncated  edges  of  the  inclined 
layers  in  the  medial  portion.  The  apex  of  the  cone  is  well 
up  the  feeding  stream  from  the  locality  where  the  delta  first  < 
began,  and  as  the  delta  increases  in  size,  migrates  up  stream 
at  the  same  time  that  the  alluvial  cone  increases  in  all  of 
its  dimensions.  '■ 

The  feeding  stream  in  flowing  down  the  surface  of  the 
delta  cap  is  deflected,  or  caused  to  divide,  from  time  to 
time,  on  account  of  the  deposits  made  on  its  bottom  and 
sides,  and  sends  off  branches  in  the  same  manner  as  has 
been  explained  in  the  case  of  alluvial  cones,  and  at  one 
time  or  another  flows  over  all  portions  of  the  surface  of  the 
delta  and  discharges  at  all  points  about  its  outer  border.  -^ 
The  delta  cap  is  thus  built  up,  and  the  periphery  of  its  base 


if^ 


4 

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t 

i 

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128 


RIVERS  OF  NORTH  AMERICA 


extended  by  the  addition  of  material  to  its  surface.  As  in 
the  case  of  alluvial  cones,  the  surface  slope  of  the  delta  cap 
is  controlled  by  the  grade  of  the  feeding  stream,  but  be- 
comes less  and  less  steep  as  the  area  of  its  base  increases. 
The  more  or  less  definite  layers  of  which  it  is  composed  are 
inclined  and  slope  downward  towards  the  receiving  water- 
body.  As  each  layer  was  added  to  its  surface  at  a  certain 
stage  in  its  growth,  the  inclination  of  those  first  formed  is 
somewhat  greater  than  the  slope  of  those  deposited  later. 
An  inclination  of  three  to  five  degrees  with  a  horizontal 
plane  is  common  in  deltas  of  the  class  here  considered.  If 
we  visit  a  growing  delta  in  process  of  construction  by  a 
high-grade  stream,  it  will  be  found  that  the  stream  is  bring- 
ing both  coarse  and  fine  material  to  the  apex  of  the  delta 
cap.  The  coarser  portions  of  this  load  are  dropped  on  the 
exposed  surface  of  the  delta,  together  with  much  fine  ma- 
terial which  serves  to  fill  the  interspaces  between  the  boul- 
ders and  larger  stones  and  tends  to  prevent  loss  of  water 
by  percolation.  This  building  up  or  aggrading  serves  to 
adjust  the  grade  of  the  feeding  stream  as  its  length  increases 
owing  to  the  outward  extension  of  the  delta.  Both  coarse 
and  fine  material  is  carried  down  the  slope  of  the  delta  cap 
and  delivered  to  the  receiving  water-body,  where  it  is 
assorted,  the  larger  and  heavier  debris  falling  at  once  and 
adding  to  the  series  of  inclined  layers  previously  deposited, 
while  the  finer  material  is  carried  beyond  the  periphery  of 
the  delta  and  distributed  over  the  adjacent  bottom  or 
floated  away,  as  already  stated,  and  finally  brought  to  rest 
with  other  fine  sediment  on  tne  floor  of  the  basin  occupied 
by  the  still  waters. 


ipied 


STREAM  DEPOSITS 


129 


In  the  passage  of  the  debris  down  the  surface  of  the  delta 
cap  an  assorting  in  reference  to  form  may  frequently  be 
recognised.  The  flat  and  angular  stones  come  to  rest  under 
conditions  that  allow  rounded  and  well-worn  stones  of  simi- 
lar size  and  density  to  be  rolled  along.  The  highly  inclined 
layers  in  a  delta  are  usually  composed  of  well-rounded 
stones. 

Referring  once  more  to  the  radial  section  of  a  delta  given 
above,  it  will  be  understood  that  the  upper  portion  or  delta 
cap  consists  of  gently  sloping,  cross-stratified,  and  irregular 
beds  of  both  coarse  and  fine  material,  the  larger  stones  being 
frequently  angular;  the  medial  member  is  composed  of 
steeply  inclined  layers,  usually  of  well-rounded  stones, 
mainly  gravel  and  sand ;  the  basal  member  is  of  fine  sand 
and  clay,  deposited  originally  in  nearly  horizontal  sheets 
which  decreased  in  thickness  in  a  direction  opposite  to  the 
source  of  supply,  but  later  were  pressed  into  folds,  or  broken 
and  displaced,  owing  to  the  heavy  weight  imposed  upon  them. 

It  should  be  noted  that  the  growth  of  a  delta  increases 
the  length  of  the  supplying  stream,  and  in  order  that  it  may 
carry  debris  to  its  advancing  terminus,  the  newly  occupied 
territory  must  be  built  up  or  aggraded.  This  process  of 
filling  in  causes  the  apex  of  the  alluvial  cap  to  migrate  up 
stream. 

At  an  advanced  stage  in  the  building  of  a  delta,  the 
lengthening  of  the  course  of  the  feeding  stream  would  de- 
crease its  rate  of  flow,  and  at  the  same  time  the  deepening 
of  the  stream's  channel  above  the  delta,  owing  to  corrasion, 
would  tend  in  the  same  direction.  These  changes  would 
necessitate  a  readjustment  of  grade  which  might  lead  to  the 


fi 


I  ,  "^-^ 


,?' 


^1 


i 

f 

t 


130 


RIVERS  OF  NORTH  AMERICA 


cutting  of  a  channel  through  the  apex  of  the  delta  in  the 
same  manner  that  the  apexes  of  alluvial  cones  are  sometimes 
notched. 

Deltas  of  Low-Grade  Streams. — Typical  examples  of 
the  deltas  of  this  class  are  furnished  by  the  deposits  now 
being  laid  down  at  the  mouth  of  the  Mississippi,  Yukon, 
Mackenzie,  Nile,  Ganges,  and  many  other  streams  that  have 
a  low  grade  for  a  long  distance  near  their  mouths  and  de- 
liver only  ^ne  sediment  to  the  still  waters  into  which  they 
flow.  In  these  deltas  the  three  divisions  so  characteristic 
Oi  the  similar  deposits  of  high-grade  streams  cannot  be 
recognised.  All  of  the  material  delivered  by  low-grade 
streams  is  fine,  mostly  of  the  nature  of  fine  sand  and  silt, 
and  no  marked  assorting  kes  place  and  no  distinctions 
between  gently  and  steeply  inclined  layers  can  be  distin- 
guished. The  tendency  of  low-grade  and  mud-charged 
streams  is  to  make  broad  deposits  with  indefinite  borders 
rather  than  thick,  well-defined  accumulations. 

The  debris  brought  to  still  waters  by  the  class  of  streams 
here  considered,  is  dropped  as  in  the  case  of  high-grade 
streams,  but,  being  fine,  a  larger  proportion  is  carried  to  a 
distance  from  the  shore.  Much  debris  is  dropped  as  the 
incoming  stream  meets  still  water,  but  the  angle  of  repose 
for  fine  sand  and  silt  is  much  less  than  for  coarse  gravel,  and 
the  outer  slope  of  the  deposit  is  not  well  defined.  More- 
over, fine  material  is  easily  disturbed  by  currents  in  the  re- 
ceiving water-body,  which  again  is  unfavourable  for  the 
formation  of  sharply  defined  delta  margins.  Low-grade, 
muddy  streams  shoal  the  still  water  at  their  mouths  and 
gradually  form  new  land,  which,   however,  is  but  slightly 


1  ii 


STREAM  DEPOSITS 


IM 


raised  above  the  surface.  The  length  of  the  stream  is  thus 
increased,  which  necessitates  a  new  adjustment  of  the 
grade  for  a  long  distance  up  its  course.  Owing  to  the 
gentle  grade  of  the  stream,  the  changes  produced  in  this 
manner  are  not  pronounced.  As  the  length  of  the  stream 
is  extended,  the  natural  levees  are  also  prolonged.  When 
floods  occur,  breaches  are  made  in  the  levees,  and  the  stream 
divides,  and  sends  off  distributaries  which  discharge  inde- 
pendently. Each  distributary  builds  a  pair  of  embankments 
or  levees,  and  also  an  independent  delta.  The  results  of 
this  process  of  subdivision  are  well  illustrated  at  the  mouth 
of  the  Mississippi,  as  may  be  seen  from  the  accompanying 
map  (Plate  V.).  Each  of  the  finger-like  extensions  of  the 
delta  is  due  to  the  prolongation  of  a  pair  of  embankments 
into  the  Gulf,  by  each  distributary,  and  the  growth  of  a 
secondary  delta  at  its  mouth.  The  Mississippi  is  thus 
building  a  compound  delta,  composed  of  the  secondary 
deltas  formed  at  the  mouths  of  th^  several  distributaries  of 
the  main  river.  The  fact  that  each  distributary  is  forming 
a  delta  of  its  own  is  best  illustrated  perhaps  at  the  mouth 
of  what  is  known  as  Cubit's  Gap,  a  break  in  the  left  levee, 
about  four  miles  upstream  from  where  the  main  "  passes" 
or  distributaries  diverge. 

The  branches  of  the  secondary  deltas,  with  their  levees, 
frequently  joi.i,  and  the  low  spaces  between  them  become 
transformed  into  lakes.  Delta  lakes,  like  Pontchartrain  to 
the  north  of  New  Orleans,  are  thus  formed.  Several  lakes 
of  this  type  are  shown  on  the  accompanying  maps  (Fig.  5 
and  Plate  V.),  as  well  as  a  number  of  bays,  which  will  evi- 
dently be  shut  off  from  the  sea  in  time  and  become  lakes. 


,1  ■■ 


Wjf 


1^ 
P 


132 


RIVERS  OF  NORTH  AMERICA 


The  surface  of  the  delta  of  a  low-grade  stream  is  in  reality 
an  alluvial  cone,  but  of  such  a  low  slope  that  the  eye  cannot 
usually  distinguish  it  from  a  horizontal  plane.  The  exposed 
portion  of  such  a  delta,  with  its  many  levees,  corresponds, 
both  in  the  manner  of  its  formation  and  in  the  alluvial 
nature  of  its  material,  with  the  delta  cap  of  a  high-grade 
stream.  As  in  the  type  of  delta  first  considered,  the  exten- 
sion of  the  delta  increases  the  length  of  the  feeding  stream, 
and  necessitates  a  grading  up  of  the  extended  portion  of 
the  river  channel  so  as  to  furnish  the  requisite  slope  for  the 
transportation  of  debris  to  the  advancing  extremity.  The 
surface  of  the  delta  is  also  raised,  and  its  apex  migrates  up 
stream. 

The  apex  of  a  low-grade  delta  is  usually  indefinite,  and 
its  position  difficult  to  determine.  For  this  reason  in  part, 
such  a  delta  is  usually  considered  as  beginning  when  the 
first  distributary  having  an  independent  course  to  the  re- 
ceiving water-body  is  given  off.  Under  this  definition,  the 
head  of  the  delta  of  the  Mississippi  is  near  the  mouth  of 
Red  River,  or  about  two  hundred  miles  in  a  direct  line  from 
its  extreme  southern  end.  In  reality,  the  apex  of  the  delta 
cap  is  much  farther  upstream.  The  area  of  the  Mississippi 
delta,  as  determined  by  Humphreys  and  Abbot,  is  one  thou- 
sand two  hundred  and  thirty  square  miles.  The  depth  of 
the  deposit,  as  shown  by  recent  borings  at  New  Orleans,  is 
over  one  thousand  feet,'  but,  as  will  be  seen  later,  this  great 
depth  is  due  to  subsidence  and  the  superposing  of  one  delta 
on  another. 


'  E.  L.  Corthell,  "  The  Delta  of  the  Mississippi  River,"  in  The  National  Geo- 
graphic Mtigazine,  vol.  viii.,  p.  351,  1897. 


ST/iEAM   DEPOSITS 


»33 


A  delta,  comparable  in  many  ways  with  that  of  the  Mis- 
sissippi, has  been  formed  by  the  Yukon  in  Bering  Sea;  the 
distance  in  this  instance  from  where  the  first  distributary  is 
gi^'en  off  to  the  periphery  of  the  delta,  is  about  one  hundred 
miles.  The  outer  or  seaward  margin  of  the  deposit  measures 
about  seventy  miles.  The  land  between  the  several  distribu- 
taries is  swampy,  and  natural  levees  are  less  conspicuous  than 
in  the  case  of  the  Mississippi.  The  surface  of  the  delta,  a 
glimpse  of  which  is  given  in  Fig.  A,  Plate  II.,  is  treeless, 
but  covered  with  a  luxuriant  growth  of  grasses,  rushes, 
and  low  flowering  annuals,  and  is  a  luxuriant  garden  of 
flowers  in  early  summer,  but  at  a  depth  of  a  few  feet  the 
soil,  as  has  previously  been  mentioned,  is  always  frozen. 
The  delta  in  fact  is  a  part  of  the  extensive  frozen  marshes, 
or  tundra,  which  border  the  shores  of  Bering  Sea  and  the 
Arctic  Ocean.  No  survey  of  this  delta  has  been  made,  but 
a  most  instructive  example  of  the  nature  of  the  deposit 
formed  by  a  heavily  loaded  river,  subjected  to  great 
inundation,  there  invites  the  student  of  geography. 

A  great  delta  is  also  being  extended  into  the  Arctic  Ocean 
by  the  Mackenzie.  The  Colorado  is  filling  in  the  Gulf  of 
California,  and  many  smaller  streams  are  making  delta  de- 
posits in  the  lakes  of  North  America.  In  each  of  these  in- 
stances some  phase  of  delta-building  is  apt  to  be  more 
prominent  than  others,  but  in  almost  any  example  that  can 
be  chosen  the  general  laws  governing  the  deposition  of 
material  brought  by  loaded  streams  when  Iheir  currents  are 
checked  by  still  water  may  be  observed. 

An  abnormal  delta,  and  one  of  interest  on  account  of  its 
novelty,  is  being  made  in  Lake  St.  Clair  by  the  St.  Clair 


^■■v;ir 


1 

■:Ji 


ft    ' 


T 


134 


KIl'EKS  OF  NORTH  AMERICA 


River.  The  river  referred  to  is  the  outlet  of  Lake  Huron, 
and  as  lakes  act  on  settling  basins,  it  would  be  expected  that 
the  stream  draining  it  would  be  clear  and  therefore  incapable 
of  forming  a  delta.  The  shore  currents  in  Lake  Huron, 
however,  bring  debris  to  the  place  of  outlet,  and  deliver  it 
to  the  draining  stream.  This  material  is  carried  to  Lake  St. 
Clair  and  there  deposited  in  a  broad  delta,  with  several  dis- 
tributaries, similar  in  some  of  its  features  to  the  delta  of 
the  Mississippi.' 

Many  of  the  large  streams  of  North  America  are  not 
making  conspicuous  deltas  for  various  reasons.  The  St. 
Lawrence,  for  example,  is  a  clear  stream,  and  therefore  has 
but  little  material  to  deposit  when  its  current  is  checked. 
Like  a  large  number  of  rivers  flowing  to  the  Atlantic, 
the  St.  Lawrence,  as  already  stated,  has  been  greatly 
affected  at  a  comparatively  recent  date  by  a  subsidence  of 
the  land,  which  has  allowed  the  ocean  to  extend  far  up  its 
valley,  and  to  submerge  the  deltas  it  may  have  previously 
formed.  Other  streams  on  the  Atlantic  border  of  the 
continent,  like  the  Hudson,  Delaware,  Susquehanna,  Poto- 
mac, and  James,  also  enter  estuaries,  but  are  not  clear 
streams.  The  absence  of  conspicuous  deltas  about  their 
mouths  is  due  to  currents  in  the  receiving  Wr.ter-bodies, 
and  to  the  recency  of  the  drowning  of  their  lower 
courses. 

Effects  of  Changes  in  the  Elevation  of  the  Land  on  ihe 
Growth  of  Deltas. — A  rise  or  a  subsidence  of  the  land  along 
the  ocean's  'ore  where  deltas  are  being  formed,  has  the 
same  effect  on  their  growth  as  the  lowering  or  raising  of 

'  I.  C.  Russell,  Lalifs  of  North  Amtrica,  p.  40.     (linn  &  Co.,  Boston,  1895. 


STREAM  DEPOSITS 


135 


the  surface  of  a  lake  where  similar  additions  to  the  land  are 
being  made. 

If  the  surface  of  a  lake  rises  after  the  building  of  a  delta 
on  its  borders  is  well  under  way,  the  deposit  may  be  par- 
tially or  wholly  submerged  and  a  new  structure  of  similar 
character  formed  above  it.  If  the  waters  of  a  lake  subside 
after  a  tributary  stream  has  built  a  delta,  its  outer  or  sub- 
lacustral  escarpment  will  be  more  or  less  completely  ex- 
posed.  The  waters  of  the  stream  will  descend  this  slope 
with  accelerated  velocity  and  corrade.  Of  the  two  or  more 
rubdivisions  into  which  the  stream  may  be  divided  on  the 
delta  cap,  previous  to  the  change  just  assumed,  some  one 
branch,  either  by  being  shorter  than  the  others,  or  by  hav- 
ing a  greater  volume,  will  deepen  its  channel  more  rapidly 
than  its  competitor  and  draw  off  its  water,  so  that  but  one 
trench  will  be  cut  in  the  emerged  delta.  If  the  surface 
of  the  lake  is  depressed  below  the  level  of  the  base  of  the 
delta  it  v/ill  be  completely  cut  through  so  as  to  expose  a 
section  of  the  deposit  and  a  portion  of  the  material  on 
which  it  rests.  The  material  removed  during  the  cutting 
of  the  gorge,  together  with  fresh  debris  brought  by  the 
feeding  stream  from  above  the  old  delta,  will  be  again  de- 
posited and  a  new  delta  built  at  a  lower  level.  The  apex 
of  the  second  dolta  will  be  in  the  gorge  cut  through 
its  predecessor  or  below  the  base  of  the  old  delta  accord- 
ing to  the  extent  principally  to  which  the  lake  surface  is 
lowered. 

Variations  in  the  stages  of  delta-building,  owing  to 
changes  in  the  elevation  of  the  land,  or  of  the  level  of  a 
lake,  as  well  as  modifications  of  the  process  which  would 


■y 


in 


!'^ 


l|6 


mVEJiS  OF  NORTH  AMERICA 


follow  a  change  in  grade,  or  in  the  volume  or  load  of  the 
feeding  stream,  may  be  readily  determined. 

VARIATIONS   IN   NORMAL   SIKEAM   DEPOSITIONS 

[n  the  preceding  portion  of  this  chapter  we  have  c  in- 
sidered,  for  the  most  part,  the  various  processes  by  whirl' 
streams  lay  aside  their  burdens  when  not  influenced  by  dis- 
turbing conditions.  Let  us  glance  at  what  may  be  termed 
the  accidents  that  sometimes  r^odify  or  interrupt  the  normal 
processes  of  stream  deposition. 

Influence  of  Elevation  ind  Depression  of  the  Latid  on 
Stream  Deposition. — The  velocity  of  a  strean.,  as  we  have 
seen,  has  a  controlling  influence  on  the  amount  of  debris  it 
can  transport.  A  change  in  condil  ns  which  will  increase 
the  velocity  of  a  stream,  other  conditions  remaining  the 
same,  will  increase  its  transporting  power.  The  reverse  of 
this  proposition  is  also  true. 

Movements  of  the  'and  both  of  the  nature  of  elevation 
and  depression  are  known  to  be  in  progress  in  many  r^'gions 
and,  we  have  good  reasons  for  believing,  have  at  one  time 
or  another  affected  every  portion  of  the  earth's  surface. 
These  movements,  due  to  changes  in  the  interior  of  the 
earth,  fn  maiiy  instances  affect  the  surface  in  such  a  manner 
as  to  tf^t  broad  areas.  Such  tilting  furnishes  an  exit^yl'" 
of  the  simplest  movements  of  the  earth's  crust,  so  far  as  the 
changes  here  considered  affect  the  flow  of  streams. 

When  the  land  is  tilted  downward  in  the  direction  in 
which  a  stream  flo^ws,  the  velocity  of  the  stream  will  evi- 
dently be  increased  and  its  energy  available  for  corrasion 
and  transportation  also  increased.     If,  however,  the  rocks 


STREAxM  DEPOSITS 


m 


underlying  the  hydrDgraphic  basin  of  a  river  are  tilted 
downward  in  a  direction  opposite  to  the  flow  of  the  stream, 
its  velocity  will  be  decreased,  and  its  corrading  and  trans- 
porting power  lessened. 

It  is  thus  evident  that  when  the  land  is  tilted  so  as  to 
favour  corrasion,  stream  channels  will  be  deepened  at  a  more 
rapid  rate  than  previous  to  the  change;  and  when  the  tilting 
is  in  ;5uch  a  direction  as  to  check  the  flow  of  streams,  other 
conditions  remaining  the  same,  corrasion  will  decrease  and 
depocltion  may  take  place  and  the  stream  valleys  be  ag- 
graded. Areas  in  the  course  of  a  stream  which  are  de- 
pressed with  reference  to  adjacent  areas  will  receive  deposits 
and  be  filled  until  the  normal  relation  of  stream  bed  to 
current  has  been  re-established.  An  illustration  of  this 
process  is  furnished  in  Southern  Washington,  at  the  west 
base  of  the  Blue  Mountains,  where  movements  in  blocks  of 
the  earths  crust  are  producing  a  depression  of  an  area  some 
twenty  miles  in  diameter,  with  reference  to  adjacent  areas. 
The  streams  from  the  Blue  Mountains  descend  steep  d  "clivi- 
tics,  are  swift,  and  bring  large  quantities  of  debris  to  the 
depressed  area.  Deposition  is  there  in  progress  and  a  gravel 
plain  is  in  process  of  formation.  Ihp  streams  in  crossing 
the  area  wuere  aggrading  is  under  way  bifurcate  much  the 
same  as  on  a  delfa,  but  the  waters  unite  to  form  a  single 
stre<»m.  Walla  Walla  River,  where  the  aMa  of  relative  dc- 
)n  is  psMsed.' 

Infiueme  of  Variations  in  Load  on  Stream  Deposition. — It 
has  been  d«»monstratcd  that  a  stream  of  given  velocity  and 

'  I,  C-  Rkwi*ll,  "  Keconnoiuance  in  Southeastern  Washington,"  U,  S.  Get- 
logitai  Surtax,  yVater  Supply  and  Irrigation  Paftrs,  No.  4,  pp.  3i,  34,  1897. 


1- 


IggggSl 


138 


RIVERS  OF  NORTH  AMERICA 


V:' 


of  stated  volume  has  a  certain  competency  to  transport 
debris.  If  the  quantity  of  debris  dehvered  to  a  stream  ex- 
ceeds Its  competency,  a  part  is  dropped  and  a  part  carried 
on.  Under  such  conditions,  a  selective  power  is  manifest, 
the  stream  dropping  the  larger  and  heavier  debris  and 
carrying  on  the  smaller  or  lighter.  Changes  in  velocity,  as 
from  swift  to  sluggish  reaches  in  a  stream,  lead  to  the  drop- 
ping of  debris  when  the  current  slackens,  and  a  consequent 
aggrading,  which  tends  to  give  the  stream  channel  a  uniform 
slope.  If  debris  is  added  to  a  stream  already  partially 
loaded,  the  result  is  much  the  same  as  if  its  velocity  was 
checked.  If  the  debris  added  is  in  excess  of  the  compe- 
tency of  the  stream,  the  coarser  material  is  dropped  and  the 
finer,  up  to  a  certain  grade,  carried  on. 

The  principal  results  of  this  law  are  seen  where  high- 
grade  and  consequently  swift  tributaries  join  a  low-grade 
trunk  stream.  The  tributaries  may  under  such  conditions 
bring  more  debris  to  the  main  stream  than  it  is  competent 
to  carry  on,  and  a  deposit  is  made.  The  result  is  thai  an 
obstruction  is  foimed  in  the  main  stream  at  the  mouth  of 
each  high-grade  tributary  and  the  waters  above  are  more  or 
less  completely  ponded.  Lakes  are  sometimes  formed  in 
trunk  streams  for  this  cause.  Lake  Pepin,  for  example,  is 
held  in  chock  by  debris  brought  to  the  Mississippi  by  Chip- 
pewa River,  in  excess  of  the  amount  the  receiving  stream 
is  able  to  remove.  In  the  Colorado,  as  described  by  Powell, 
rapids  due  to  a  similar  cause  occur  just  below  the  mouths  of 
several  of  its  tributaries. 

The  rate  at  which  r^treams  cor»-.tde  varies,  other  conditions 
being  the  same,  witii  the  resistance  of  the  rocks  >  .      ,    v.-^ 


E^L 


STREAM  DEPOSITS 


139 


they  flow.  When  the  rocks  are  soft  and  easily  corraded, 
the  loads  of  the  streams  are  increased  and  their  velocities 
checked.  In  such  regions,  also,  vertical  corrasion  is  fre- 
quently retarded  by  the  occurrence  of  harder  beds  farther 
down  the  streams,  while  lateral  corrasion  is  still  possible; 
the  streams  then  expand  in  the  areas  of  soft  rock  and  may 
form  broad  flood-plains. 

The  loads  of  streams  are  also  increased,  as  previously 
stated,  by  the  action  of  the  wind  in  bringing  sand  and  dust 
to  th-^m.  When  this  occurs,  the  tendency  is  much  the 
same  as  when  a  high-grade  tributary  delivers  more  debris 
than  the  stream  can  bear  away.  Trains  of  sand  dunes 
travel  over  many  regions  in  the  direction  of  the  prevailing 
wind.  If  a  train  of  dunes  reaches  a  river,  the  load  of  the 
stream  is  increased,  and  may  exceed  its  capacity  to  transport 
and  a  dam  be  formed.  The  waters  may  rise  above  such  a 
dam  and  an  increase  of  velocity  be  secured  which  will  lead  to 
the  removal  of  the  obstruction  either  wholly  or  in  part.  In 
many  instances,  a  struggle  ensues  between  the  winds  bring- 
ing debris  to  a  stream,  and  the  water  striving  to  remove  it. 
If  the  supply  of  wind-borne  debris  is  sufficient,  a  dam  is 
formed,  and  may  be  raised  \  r'l  ab.  ve  the  level  of  the  lake 
that  it  holds  in  check.  The  waters  of  the  lake  may  then 
rise  until  a  balance  between  inflow  and  loss  by  evaporation 
is  established,  and  an  enclosed  lake,  that  is,  one  without  an 
outlet,  is  formed.  An  example  of  a  water  body  held  in 
check  by  sand  dunes  is  furnished  by  Moses  I.akc  in  the 
central  part  of  Washington.  In  this  instance  the  waters 
of  the  lake  escape  in  part  by  percolating  through  the  sand 
dunes  retaining  them.     The  influence  of  drifting  sand  on  the 


\\ 


''.:f-V'''' 


ir 


140 


RIVERS  OF  NORTH  AMERICA 


flow  of  streams  is  more  pronounced  in  arid  than  in  humid 
regions,  for  many  reasons  which  will  suggest  themselves  to 
the  reader. 

Another  way  in  which  the  loads  of  streams  are  varied, 
is  by  glacial  action.  The  streams  flowing  from  the  ends  of 
alpine  glaciers,  frequently  receive  greater  contributions  of 
both  coarse  and  fine  material  than  they  are  able  to  bear 
away,  and  consequently  are  engaged  in  filling  in  their  val- 
leys. This  is  true  of  every  one  of  the  hundreds  of  glaciers 
in  the  valleys  of  the  Cordilleran  region  that  have  come 
under  the  writer's  notice.  Should  these  glaciers  melt, 
the  streams  flowing  from  then  and  now  aggrading  their 
valleys,  would  be  able  to  resume  the  work  of  excavation, 
and  channels  would  be  cut  through  the  deposits  of  debris 
over  which  they  now  meander. 

During  the  Glacial  epoch,  when  half  of  North  America 
was  covered  by  ice-sheets,  the  streams  fed  by  the  melting 
of  the  ice  were  greatly  overloaded  and  their  valleys  conse- 
quently deeply  filled.  Now  that  the  ice-sheets  have  melted, 
the  streams  are  at  work  in  removing  the  loads  they  pre- 
viously laid  aside. 

Influence  of  Changes  of  Climate  on  Stream  Deposition, — 
The  elements  of  climate  which  exert  the  most  direct  and 
important  influences  on  stream  depositun  are  precipitation, 
evaporation,  and  temperature. 

Of  these,  precipitation  is  bj-  far  the  most  important. 
Any  change  in  the  amount  of  rain  fall,  or  hi  ItH  distribution 
throughout  the  year,  is  at  once  felt  by  the  streams  In  the 
region  affected. 

Evaporation  depends  on  temperature  and  on  the  strength 


STREAM  DEPOSITS 


141 


•th 


of  the  wind,  and  tends  to  diminish  the  volume  of  streams 
throughout  their  entire  length. 

Temperature  exerts  a  varied  influence.  A  high  mean  an- 
nual temperature  favours  evaporation  from  the  ocean,  es- 
pecially, and  except  under  certain  local  conditions  insures 
abundant  rain-fall  and  favours  also  the  growth  of  vegetation, 
thereby  increasing  the  supply  of  organic  acid  available  for 
surface  water.  Warm  water  charged  with  organic  acid  pro- 
motes rock  decay  and  thus  favours  the  preparation  of  debris 
for  transportation.  A  low  temperature,  on  the  other  hand, 
not  only  reverses  the  conditions  just  named,  but  when  the  de- 
crease is  sufficient  to  cause  the  freezing  of  water,  rain  changes 
to  snow,  the  ground  is  frozen,  and  percolation  ceases.  The 
storing  of  the  winter's  precipitation  in  the  form  of  snow 
and  ice,  however,  favours  stream  work  and  the  general  degra- 
dation of  the  land,  byconcentratingtheenergy  of  the  streams 
at  the  time  the  snow  melts.  The  freezing  of  water  in  the 
interstices  of  rock,  as  previously  mentioned,  is  one  of  the 
most  powerful  agencies  tending  toward  rock  disintegration. 
In  these  and  still  other  ways  climatic  conditions  exert  an 
influence  either  directly  or  indirectly  on  stream  deposition, 
but  at  present  it  will  be  most  profitable  to  confine  attention 
to  variations  in  volume  due  to  changes  in  supply. 

In  g^ncrc^l,  as  is  well  known,  a  decrease  in  the  rain-fall  of 
a  given  region  is  accompanied  by  a  decrease  in  the  volume  of 
the  draining  streams,  and  consccjuently  a  loss  in  their  trans- 
porting power.  The  behaviour  of  streams  under  such  con- 
ditions is  materially  influenced  by  the  rate  at  which  they 
are  supplied  with  debris.  During  heavy  rains  a  stream  may 
be  overloaded  and  caused  to  deposit,  in  spite  of  its  increased 


\\ 


142 


RIVERS  OF  NORTH  AMERICA 


velocity  due  to  greater  volume,  and  the  amount  of  work 
done  in  a  given  time  is  far  in  excess  of  that  accomplished 
during  an  equal  time  when  the  precipitation  is  less. 

The  influence  of  variations  in  precipitation  is  illustrated 
by  the  annual  change  in  streams  during  rainy  and  dry  sea- 
sons. During  rainy  seasons,  more  especially  in  spring  in 
temperate  latitudes,  when  the  rain  causes  the  melting  of 
previously  accumulated  snow,  the  streams  are  swollen  and 
heavily  charged  with  debris.  They  overspread  their  banks 
and  deposit  material  on  their  flood-plains.  It  is  during  the 
time  the  streams  overflow  their  banks  that  the  greater 
amount  of  material  is  deposited.  Much  debris  is  also  laid 
down  at  such  times,  however,  in  the  stream  beds,  even 
when  the  current  is  swift,  and  in  some  instances  the  less 
heavily  loaded  water,  when  much  decreased  in  volume, 
corrades  channels  through  deposits  made  during  high-water 
stage.  This  may  be  seen  in  many  wayside  rills,  whicli 
spread  out  in  sheets,  heavily  charged  with  debris,  during 
storms,  and  make  deposits  through  which  the  shrunken 
and  less  heavily  charged  rills  at  a  later  stage  excavate 
channels. 


THE   GENERA',    PROCESS   OF   STRE/VM   CORRASION  AND 

DEPOSITION 


i 


The  action  of  streams  in  corrading,  transporting,  and  de- 
positing debris  is  such  a  complex  process  that  it  is  con- 
venient to  consider  the  different  phases  of  their  work 
separately.  For  this  reason,  an  effort  has  been  made  in 
this  chapter  to  direct  special  attention  to  the  manner  in 
which  streams  lay  aside  their  loads  during  the  process  of 


STREAM  DEPOSITS 


HZ 


development  that  they  pass  through.  The  behaviour  of 
streams  is  much  like  the  action  of  a  complex  piece  of 
machinery,  as  a  watch,  for  example;  changes  cannot  be 
made  in  one  portion  of  the  mechanism  without  affecting  the 
action  of  the  whole,  and  necessitating  adjustments  through- 
out. Considering  deposition  alone,  we  find  that  streams  in 
general,  in  passing  from  a  high  to  a  low  grade,  make  de- 
posits, as  where  a  river  leaves  its  mountain  tract  and  enters 
a  valley  tract,  or  passes  from  a  swift  to  a  more  qr  et  reach. 
Streams  subject  to  floods  make  deposits  over  the  lands  they 
inundate  during  high-water  stages,  and  spread  out  flood- 
plains.  At  such  times,  also,  the  heaviest  deposit  is  in  the 
immediate  border  of  the  low-water  channel,  and  natural 
levees  are  built.  A  high-grade  stream,  tributary  to  a  low- 
grade  and  consequently  less  swift  stream,  unless  the  differ- 
ence in  grade  is  more  than  counterbalanced  by  the  volume 
o\  the  receiving  stream,  makes  deposits  and  the  waters  of 
the  main  stream  are  more  or  less  complet«:ly  ponded. 
Local  overloading  from  the  action  of  the  wind  or  of 
glaciers  produces  similar  results. 

The  debris  carried  in  suspension  by  streams  or  rolled 
along  their  beds  is  also  deposited  in  lakes  as  deltas,  or  dis- 
tributed over  their  bottom.  As  lakes  in  many  instances  are 
of  the  nature  of  expansions  of  streams,  the  filling  of  their 
basins  may  be  considered  as  a  part  of  the  general  process  of 
stream  deposition  by  which  stream  channels  are  aggraded. 
In  discussing  corrasion  it  was  shown  that  a  stream,  at  least 
in  humid  climates,  cuts  down  its  channel  to  baselevcl  most 
quickly  at  its  mouth,  and  that  the  process  of  deepening  pro- 
gresses up  stream.      The  head-waters  of  a  well-developed 


i- 


r. 

144 


RIVERS  OF  NORTH  AMERICA 


stream  are  steeper  than  the  lower  portion  of  its  trunk.  A 
general  view  of  stream  deposition  shows  that  a  similar  order 
is  followed  in  the  process  of  stream  deposition.  When  the 
seaward  portion  of  the  trunk  of  a  stream  has  been  lowered 
to  baselevel,  the  stream  continues  to  corrade  laterally,  and 
thus  makes  it  possible  for  flood-plains  to  form.  As  a 
stream  continues  to  widen  its  channel  farther  and  farther 
from  its  mouth,  the  flood- plain  follows.  If  a  stream 
is  making  a  delta,  its  length  of  flow  is  increased,  and  its 
flood-plains  and  channel  are  raised  by  deposition  in  order 
to  furnish  the  necessary  slope.  When  a  stream  reaches 
maturity,  its  plains  tract  and  valley  tract  are  greatly  length- 
ened at  the  expense  of  the  high-grade  portions  of  its  course 
in  the  uplands.  The  high-grade  branches,  then,  bring 
more  material  than  the  trunk  stream  can  bear  away,  and 
the  flood-plains  along  its  sides  are  raised  by  the  de- 
position of  material  laid  aside.  During  the  upbuilding 
of  the  flood  -  plains  the  stream  channel  is  also  raised. 
Hence,  for  a  long  time  after  a  normal  river  has  cut  down 
its  channel  in  its  lower  course  practically  to  baselevel, 
building  is  in  progress  and  the  valley  becomes  filled  in  with 
abandoned  debris.  There  comes  a  time,  however,  when  the 
highlands  from  which  the  river  flows  have  been  lowered 
so  that  the  branches  of  the  mam  stream  are  not  as  swift  as 
previously,  and  the  stream  is  enabled  to  devote  a  portion 
of  its  energy  not  consumed  in  the  friction  of  flow  to  the  re- 
excavation  of  its  channel  farther  seaward.  As  this  process 
is  continued,  the  highest  flood-plain  is  abandoned  and  new 
ones  formed  at  lower  levels,  thus  giving  origin  to  terraces, 
as  will  be  shown  in  the  next  chapter.     During  this  stage  of 


STREAM  DEPOSITS 


145 


a  stream's  development,  as  in  the  preceding  stages,  changes 
occur,  also,  in  the  longitudinal  profile  of  the  stream 
throughout  its  length. 

The  manner  in  which  flood-plains  are  formed  and  advance 
up  stream  as  the  down  cutting  of  the  upper  portion  of  a 
stream  channel  pn egresses,  shows  that  only  an  approximation 
to  baselevelling  is  reached  during  the  earlier  stages  of  a 
stream's  development.  It  is  after  a  stream  has  lowered  its 
head-water  channels  so  as  to  permit  of  the  removal  of  the 
flood-plains  built  lower  downstream,  that  what  may  be 
termed  a  second  approximation  to  baselevel  is  normally 
reached. 


PROFILES   OF   STREAMS 

In  a  discussion  of  the  succession  of  changes  experienced 
by  a  stream  during  its  life,  consideration  should  be  given  to 
the  orderly  variations  in  shape  that  occur  in  the  valley  it 
excavates. 

The  shape  of  a  valley  may  be  illustrated  by  two  classes  of 
profiles;  one  longitudinal  and  the  other  transverse.  A  gen- 
eralised longitudinal  profile  of  a  stream  would  be  what  is 
understood  as  a  projection  on  a  vertical  plane;  that  is,  it  is 
approximately  the  profile  which  the  stream  would  have  if  it 
flowed  in  a  perfectly  straight  course  from  source  to  mouth. 
Such  a  profile,  together  with  a  sufficient  number  of  cross- 
profiles,  would  enible  one  to  construct  a  model  of  a  valley 
showing  the  actual  relations  c.nd  proportions  of  its  several 

pu.i  CS. 

'7/^  Longitudinal  Profile. — A  young  stream  flowing  down 
the  surface  of  a  tilted  plain,  we  will  assume,  will  necessarily 


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RIVERS  OF  NORTH  AMERICA 


have  the  same  gradient  as  the  land  wh '.ch  gave  direction  to 
its  current.  As  such  a  stream  entrenches  itself  by  corrading 
the  bottom  of  its  channel,  and  during  the  process  of  cutting 
down  to  baselevel  spreads  out  flood-phins,  which  are  sub- 
sequently dissected,  it  will  develop  i.  series  of  profiles  to 
suit  the  various  stages  of  its  development. 

An  ideal  example  of  the  succession  of  longitudinal  profiles 
which  a  stream  makes,  may  be  had  by  assuming  that  it 
works  in  homogeneous  rocks  throughout  its  course  and  is 
not  disturbed  by  changes  ot  climate,  the  formation  of 
glaciers,  or  other  modifying  conditions.  When  the  typical 
profile  of  a  young  stream  and,  the  changes  it  passes  through 
as  the  stream  advances  in  its  appointed  task  is  understood, 
the  modifications  due  to  climatic  and  other  disturbances,  or 
accidents,  as  they  may  bt  termed,  can  be  readily  recognised. 

As  has  been  shown  with  exceptional  clearness  by  Hicks,' 
corrading  streams  have  curved  profiles,  the  curvature  being 
concave  upward,  while  deposits  laid  down  by  currents,  such 
as  alluvial  cones,  have  a  reverse  curvature,  that  is,  they  are 
convex  to  the  sk) .  The  longitudinal  profile  of  a  stream 
which  is  corrading  in  its  mountain  tract  and  spreading  out  a 
flood-plain  farthe*-  down  its  course,  must  therefore  have  a 
double  curvature — that  is,  it  »vill  be  concave  in  its  upper 
course  but  convex  in  its  lower  course.  The  concave  portion 
of  the  curve  is  much  more  conspicuous  than  the  more  gentle 
curve  due  to  deposition,  and  it  is  frequently  stated  that  the 
profile  of  a  stream  is  a  concave  curve  throughout  its 
length.     This,  however,  can  only  be  strictly  true  when  a 

•  L.  E.  Hicks,  "  Some  Elements  of  Land  Sculpture,"  in  Bulletin  0/ the  Gfu- 
logical  Society  0/  Am friia,  vol.  iv.,  pp.  133-146,  1893. 


STREAM  DEPOSITS 


H7 


Stream  is  engaged  in  corrading  Its  channel  from  source  to 
mouth. 

A  generalised  profile  of  a  stream  which  is  corrading  its 
channel  throughout  is  shown  approximately  in  the  following 
diagram.     It  will  be  noticed  that  the  curvature  iS  compara- 


FlG.  7.     Longitudinal  Profile  of  a  Young  Stream. 

tively  great  near  the  source  of  the  stream,  but  decreases  and 
becomes  nearly  horizontal  on  approaching  its  mouth.  There 
is  a  suggestive  resemblance  between  such  a  profile  and 
cycloid  curves.  As  is  well  known,  a  cycloid  curve  is  the 
curve  of  quickest  descent  for  a  body  moving  from  a  given 
point  to  a  lower  one  not  in  the  same  vertical  line.  Should 
accurate  survey  show  that  streams  corrading  homogeneous 
rocks  actually  produce  cycloid  curves,  or  the  curves  of 
quickest  descent  for  their  debris-charged  waters,  it  will  fur- 
nish another  illustration  of  the  wonderful  harmony  that  pre- 
vails in  nature.  A  stream  in  cutting  down  its  channel  to 
baselevel  must  evidently  reach  that  limit  first  at  its  mouth, 
and  will  then  continue  to  deepen  its  bed  progressively  up 
stream.  If  this  operation  should  be  allowed  to  go  on  with- 
out deposition  and  the  formation  of  flood-plains,  the  result 
would  evidently  be  the  flattening  of  the  curved  profile  from 
the  mouth  of  the  stream  to  its  source  at  the  same  time  that 
the  elevation  of  the  stream  channel  above  sea-level  was 
lowered  progressively  and  in  an  increasing  ratio  from  mouth 
to  source.     Corrision,   however,  is  accompanied  by  sedi- 


n 

'i 
-* 


148 


RIVERS  OF  NORTH  AMERICA 


pn 


mentation.  In  young  streams,  ccrrasion  may  occur  through- 
out their  courses,  but  as  soon  as  their  mouths  are  lowered 
to  baselevel,  deposition  begins  and  progressively  advances' 
up  stream.  The  longitudinal  profiles  of  most  streams  result 
from  both  corrasion  and  sedimentation,  and  have  a  double 
curvature.  Corrasion  is  more  active  in  the  mountain  tract 
than  in  the  valley  and  plains  tracts,  and  until  these  divisions 
are  obliterated  by  advancing  age,  the  profile  of  a  stream  is, 
in  part,  due  to  corrasion  and  in  part  to  sedimentation. 
With  advancing  age  the  portion  of  the  curve  due  to  deposi- 
tion advances  up  stream  at  the  expense  of  the  steeper 
portions  of  the  profile  where  corrasion  is  still  in  progress. 
There  comes  a  time  in  the  development  of  a  stream,  how- 
ever, when  this  advance  is  checked,  and  when  the  flood-plain 
deposits  begin  to  be  dissected  ;  the  swing  is  then  the  other 
way,  and  the  portion  of  the  profile  due  to  corrasion  is 
lengthened  and  progresses  toward  the  mouth  of  the  stream. 
In  old  age  the  profiles  of  streams  become  flattened  and  ap- 
proach more  and  more  nearly  a  straight  line,  but  probably 
never  reach  that  condition. 


r 


Fig.  8.     Successive  Changes  in  the  Profile  of  a  Divide  Owing  to  Corrasion  and 
Weathering  :  Vert.'cal  Scale  Exaggerated. 

The  he.ivy  broken  line  indicates  the  profile  cf  an  uplift  an  it  might  appear  had  there  been  no 
erosion  ;  the  smaller  bn.ken  lines  show  weather-curves  ;  the  dotted  lines,  successive  cor- 
nuion  curves  ;  and  the  solid  curved  line  below,  the  profile  of  the  resulting  old-land  surface. 

In  the  above  diagram  an  attempt  is  made  to  show  quali- 
tatively the  successive  changes  that  the  profiles  of  streams 
pass  through  from  youth  to  old  age.     In  the  cise  assumed, 


STREAM  DEPOSITS 


140 


two  streams  flow  in  opposite  directions  from  a  common 
divide,  and  are  so  nicely  balanced  against  each  other  that 
the  divide  has  been  lowered  in  a  single  vertical  plane.  The 
concave  curvature  of  the  profiles  in  their  upper  courses  in- 
creases during  early  youth,  reaches  its  maximum  when  the 
streams  are  mature,  and  then  decreases  with  advancing  age. 
On  account  of  the  exceedingly  gentle  concave  curves  due  to 
deposition,  it  is  impossible  to  represent  them  on  the  scale 
here  used. 

When  the  profiles  of  oppositely  flowing  streams  meet  at 
the  crest  of  a  mountain  range,  there  should  be,  if  no  modify- 
ing conditions  intervene,  a  sharp  divide,  as  is  indicated  in 
Fig.  9.  On  some  mountain  cr^-rts  this  condition  is  very 
nearly  reached.     As  one  follows  up  a  stream  and  approaches 


Fig.  9.     Ideal  Profile  of  a  Divide  between  the  Head-Waters  of  Two  Opposite- 
Flowing  Streams  :  Vertical  Scale  Exaggerated. 

its  ultimate  source,  the  rate  of  corrasion  progressively  di- 
minishes, for  the  reason  that  the  water  supply  becomes 
f.maller  and  smaller.  The  rocks,  however,  are  everywhere 
exposed  to  the  denuding  agencies  of  the  air,  namely,  rain, 
wind,  frost,  etc.,  and  at  the  heads  of  drainage  lines  the 
action  of  these  agencies  is  in  excess  of  stream  corrasion,  and 
convex  curves  due  to  weathering  modify  or  replace  the  con- 
cave curves  due  to  stream  action.  Unless  the  rocks  on  a 
divide  between  two  drainage  systeiTJs  which  head  against 
rach  other  are  unusually  resistant,  and  maintain  angular 
forms  as  they  weather,  the  concave  profile  leading  up  to  the 


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150 


HI  VERS  OF  NORTH  AMERICA 


divide  from  either  side  changes  to  convex  curves  before 
uniting.'  The  usual  profile  in  such  instances  is  shown  in 
the  lower  curves  in  Fig.  8.  - 

Cross-Profiles. — The  cross-profiles  of  stream-cut  valleys 
change  in  the  same  locality  with  the  age  of  the  stream,  and 
are  modified  by  the  weathering  of  the  valley  sides,  the 
texture  of  the  rocks,  etc.  If,  as  above,  we  conceive  of  a 
valley  being  cut  out  of  homogeneous  rocks  and  ascertain 
what  changes  in  its  cross-profile  at  a  given  locality  will  result 
from  stream  action  and  weathering,  the  modifications  due 
to  other  causes  may  be  more  easily  recognised. 

A  young  and  rapidly  corrading  stream  working  in  moder- 
ately hard  rocks  produces  a  gorge  or  canyon  with  steep 
sides.  The  cross-profile  of  such  a  gorge  is  markedly  V- 
shaped,  except  that  the  bottom  of  the  V  is  slightly 
rounded.  The  width  of  the  concave  bottom  of  the  trench 
varies  with  the  size  of  the  stream.  If  the  stream  is  work- 
ing in  hard  rocks  the  sides  of  the  trench  cut  by  it  may  be 
vertical.  ' 

As  a  stream  advances  with  its  task  of  cutting  down  its 
channel  to  baselevel,  its  rnergy  available  for  corrasion  is 
more  largely  exerted  in  the  direction  of  broadening  its  val- 
ley. The  cross-profiles  of  the  valleys  of  old  streams  be- 
come broadly  U-shaped.  The  valleys  of  streams  where  an 
approximation  to  baselevel  Y:a<,  been  reached,  or  when  flood- 
plains  are  being  formed,  generally  have  flat  bottoms  with 
more  or  less  tiaring  sides.  The  cross-profiles  then  resemble 
more  or  less  closely  the  figure  which  would  be  obtained  by 

'  L.  E.  Hicks,  "  Some  Elements  of  Land  Sculpture,"  in  Bulletin  of  the  Geo- 
logical Society  of  America,  vol.  iv.,  pp.  133-146,  1893. 


STREAM  DEPOSITS 


151 


breaking  a  plate  straight  across.  That  is,  the  bottom  is  a 
horizontal  line  bordered  by  ascending  lines.  The  graceful 
double  curves  in  the  profile  on  each  side  of  an  aggrading 
stream  have  already  been  referred  to. 

As  a  stream  advances  in  age,  the  cross-profile  at  a  given 
locality  gradually  changes  from  a  V-shape  to  a  U-shape, 
and  then  to  a  ^ — ^ -shape.  In  extreme  old  age  the  bottom 
becomes  greatly  broadened  with  reference  to  the  height  of 
the  sides. 

The  slope  of  the  sides  of  a  valley,  whatever  its  age,  de- 
pends on  the  texture  of  the  rocks  and  on  weathering.  In 
hard  rocks  the  slopes  are  steeper  than  in  soft  rocks.  If  the 
rate  at  which  a  stream  deepens  or  widens  its  valley  is  rapid 
in  reference  to  the  rate  of  weathering,  the  sides  will  be 
steep,  but  if  the  reverse  is  the  case  the  slopes  will  be  gentle. 
It  is  thus  evident  that  the  character  of  the  cross-profile  of  a 
stream-cut  valley  depends  largely  on  climate,  on  rock  text- 
ure and  rock  structure,  on  relative  rate  of  corrasion  and 
weathering,  and  on  the  stage  in  development  that  the  stream 
has  reached. 


■4i 


' 


CHAPTER  VI 

STREAM  TERRACES 

A  TERRACE  may  be  defined  as  a  step-like  area  with  a 
nearly  even  and  approximately  level  surface,  bounded 
on  one  margin  by  an  ascending  and  on  the  other  by  a 
descending  slope.  A  stairway  may  be  considered  as  an 
example  of  a  series  of  terraces  bounded  by  vertical  escarp- 
ments. In  nature  there  are  many  departures  from  the  reg- 
ularity in  form  implied  in  the  above  statements,  due  in  part 
to  the  conditions  under  which  they  were  made,  but  more 
commonly  to  subsequent  changes.  The  surface  of  a  terrace 
is  frequently  uneven,  and  cut  across  by  rill-channels  and 
gullies,  or  talus  slopes  and  landslides  may  encumber  it. 
The  bounding  slopes  may  be  steep,  or  depart  but  slightly 
from  the  horizontal.  A  cross-profile  of  a  river  valley  with 
terraces   on  each  side  is  shown  in  the  following  diagram. 


Ftg.  io.     Ideal  Cross-Profile  of  a  Terraced  Valley. 

This  figure  is  intended  simply  to  illustrate  the  general  char- 
acteristics of  stream  terraces,  and  not  to  indicate  the  precise 
conditions  which  the  student  may  expect  to  find  when  he 
supplements  his  reading  by  cross-country  tramps. 

15a 


STREAM    TERRACES 


153 


Terraces  of  this  general  character,  from  a  few  feet  to 
several  rods  broad,  may  frequently  be  traced  for  many  miles 
on  each  border  of  a  river  valley.  In  numerous  instances 
several  terraces  one  above  another  with  various  intervals  be- 
tween may  be  recognised  on  the  same  slope.  They  follow 
all  of  the  windings  of  the  valleys,  sweeping  about  prominent 
bluffs  and  into  adjacent  embayments  in  broad,  beautiful 
curves.  Much  of  the  charm  alike  of  sheltered  dells  and  of 
broad  valleys  is  frequently  due  to  the  symmetrically  curving 
lines  formed  by  the  terraces  on  the  bordering  slopes  of  the 
adjacent  uplands.  This  is  true  more  especially  of  the  valleys 
of  the  Northern  Appalachians  and  New  England  and 
thence  westward  through  the  vast  areas  formerly  occupied 
by  glaciers.  Many  of  the  valleys  in  the  mountains  of 
the  Cordilleran  region  are  also  terraced  in  a  remarkable 
manner. 

River  valleys,  as  we  know,  have  been  excavated  by  the 
streams  flowing  through  them,  and  it  is  at  once  evident 
that  the  terraces  beautifying  their  sloping  sides  must,  in 
most  instances,  be  due  to  the  same  agency.  Another  obser- 
vation confirming  this  conclusion,  is  that  the  terraces  are 
not  horizontal  when  followed  in  the  direction  of  their  lengths, 
but  have  a  gradient  similar  to  that  of  the  stream  flowing 
through  the  bottom  of  the  valley  in  which  they  occur,  but 
not  precisely  coinciding  with  it. 

The  fact  that  stream  terraces  are  not  horizontal  in  the 
direction  of  their  lengths  serves  to  distinguish  them  from 
similar  topographic  forms  made  by  the  waves  and  currents 
of  lakes  or  of  the  ocean.  The  surfaces  of  standing  water- 
bodies  are  horizontal  in  the  every-day  sense  of  the  term. 


'm 


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ff 


ill: 


154 


RIVERS  OF  NORTH  AMERICA 


and  the  terraces  made  by  such  water  bodies  on  the  land 
confining  them  are  also  horizontal. 

The  presence  of  terraces  on  the  borders  of  stream-cut 
valleys  suggests  that  they  owe  their  origin  to  the  processes 
of  corrasion  or  of  deposition  which  characterise  the  work  of 
streams.  The  study  of  the  topographic  forms  under  con- 
sideration has  shown  that  they  may  be  due  to  either  of  these 
processes,  or  to  their  combined  action.  Certain  stream  ter- 
races have  been  formed  by  excavation,  others  are  the  result 
of  deposition,  while  still  others  owe  their  existence  to  a 
combination  of  the  two  processes.  We  might  classify  them 
as  cut  terraces,  built  terraces,  and  cut-and-built,  or  com- 
pound, terraces.  Such  a  classification  has  but  little  signifi- 
cance, however,  unless  the  relation  of  the  terraces  to  the  life 
histories  of  the  streams  which  gave  them  origin  is  under- 
stood. 

When  the  life  histories  of  streams  are  reviewed,  and  the 
modifications  in  their  normal  development  due  to  climatic 
changes  and  to  secular  movements  in  the  earth's  crust  are 
considered,  it  will  be  found  that  there  are  three  principal 
causes  which  lead  to  the  origin  of  terraces.  These  are :  ist. 
the  normal  changes  in  a  stream  valley  due  to  the  successive 
processes  of  corrasion,  flood-plain  building,  and  re-excava- 
tion ;  2d.  climatic  changes  which  cause  variations  in  the 
volumes  of  streams  or  lead  to  excessive  deposition  for  a 
time,  and  the  re-excavation  of  the  partially  filled  valleys; 
and  3d.  oscillations  in  the  land  which  vary  the  rate  of  cor- 
rasion and  of  deposition.  Let  us  consider  these  three 
methods  in  the  order  named. 

Origin  of  Terraces  during  the  Process  of  Normal  Stream 


land 


Plate  VIII. 


nd  the 

imatic 

st  are 

ncipal 

1st. 

essive 

cava- 

|in  the 

for  a 

Ueys ; 

f  cor- 

three 

\tream 


Fig.  a.     Terraces  on  Fraser  River,  British  Columbia. 
Showing  post-glacial  re-excavation.     CPhotograph  by  Geological  Survey  of  Canada.) 


tils 


Fig.  B.     Terraces  in  Connecticut  Valley,  near  Bellows  Falls,  Vermont. 
(Photograph  by  C.  H.  Hitchcock.) 


•i  « 


4  m 


,#«l 


ii  I 


ill" 


J 

1 

1 
'I 

l^^fc^-    '  Lii_ 

..^. 

M^,  .' 

STREAM   TERRACES 


155 


Development. — In  discussing  the  combined  process  of  stream 
corrasion  and  deposition,  when  not  seriously  modified  by 
climatic  changes  or  movements  in  the  crust  of  the  earth, 
it  was  four.d  that  a  river  in  flowing  from  highlands  to  the 
sea  first  cuts  down  its  channel  to  baselevel  at  its  mouth 
and  then  lowers  it  progressively  up  stream,  but  during 
ils  early  life  makes  only  an  approximate  adjustment  to 
the  level  of  the  receiving  water-body.  Succeeding  the 
first  stage  of  excavation  and  following  it  progressively 
up  stream,  the  valley  is  aggraded.  This  combined  pro- 
cess is  checked  when  the  head  branches  of  the  river 
no  longer  supply  more  debris  than  the  trunk  stream  can 
carry  away,  or,  less  commonly,  when  the  course  of  the  river 
is  lengthened  by  the  formation  of  a  delta.  The  stream  then 
begins  to  excavate  a  channel  through  the  flood-plain  pre- 
viously formed.  When  this  process  of  re-excavation  begins 
the  stream  is  usually  meandering  in  broad  curves  over  a 
flood-plain.  As  the  stream  deepens  its  channel  and  sinks 
below  the  level  of  the  flood-plain,  it  retains  its  windings; 
although  the  accelerated  velocity  of  the  stream  may  tend 
appreciably  to  straighten  its  course.  When  the  stream 
lowers  its  channel,  portions  of  the  original  flood-plain  are  left 
as  terraces  on  the  sides  of  the  valley.  At  the  time  a  stream 
begins  to  deepen  its  channel,  it  may,  in  one  portion  of  its 
course,  be  in  the  centre  of  its  flood-plain,  and  will  then  leave 
a  terrace  on  each  side,  or  may  flow  on  one  Fide  or  the  other 
of  its  valley,  and  therefore  leave  a  terrace  only  on  one  border 
of  its  course.  The  stream  may  then  broaden  its  channel, 
and  spread  out  a  second  flood-plain  in  the  valley  excavated 
through  the  previously  formed  deposit. 


I' 


H..*--. 


Ife-* 


i 


Illil 


SlJ  '' 


mm 


156 


RIVERS  CF  NORTH  AMERICA 


A  stream  in  flowing  down  a  flood-plain,  it  will  be  remem- 
bered, makes  not  only  short  bends,  but  broad  sweeps  which 
carry  it  from  one  side  of  its  valley  to  the  other.  The  short 
bends  are  made  during  periods  of  time  measured  by  tons  or 
hundreds  of  years,  while  the  great  migrations  from  side  to 
side  of  a  broad  valley  require  thousands  of  years  to  com- 
plete a  single  siving.  The  short  bends  which  combine  to 
make  much  greater  curves  have  been  referred  to  in  the  case 
of  the  Mississippi,  and  may  be  readily  recognised  on  any 
good  map  of  that  river.  While  a  stream  is  deepening  its 
channel  in  a  uroad  alluvial  plain  and  building  a  second 
flood-plain  at  a  lower  level,  the  down-cutting,  between  the 
time  it  leaves  one  border  of  its  valley,  migrates  to  the  other 
side,  and  returns,  may  be  so  great  that  on  its  *eturn  it  will 
be  flov/ing  at  a  sufiiciently  lower  level  to  prevent  its  re- 
flooding  its  previously  formed  flood-plain.  When  this  hap- 
.  pens,  and  the  stream  in  its  n'igrations  does  not  swing  back 
to  its  previous  position,  a  portion  of  the  flood-plain  is  left 
and  forms  a  terrace.  Successive  terraces  may  be  left  at 
lower  and  lower  levels  by  a  contmuance  of  this  process. 

A  cross-section  of  a  valley  terraced  in  the  manner  just  de- 
scribed would  present  the  features  shown  in  the  folk  wing 
diagram.    Each  terrace  is  a  portion  of  a  flood-plain  deposit, 


Fiu.  II.     Ideal  Cross-Section  of  a  Partially  Filled  Valley  with  Terraces  Left 

during  Ke-£xc!>vaiion. 

and  the  highest  in  the  series  is  the  oldest.     The  material 
forming  the  superficial  portion  of  the  second  terrace  from 


remem- 

)s  which 

he  short 

'  tons  or 

side  to 

to  com- 

ibine  to 

the  case 

on  any 

;ning  its 

,  second 

veen  the 

he  other 

'n  it  will 

t  its  rc- 

his  hap- 

ng  back 

in  is  left 

left  at 

ess. 

just  de- 
Hewing 
oposit. 


races  Left 

katerial 
:e  from 


STREAM    TERRACES 

the  top  has  been  removed  by  the  stream,  and  re-deposited 
as  a  portion  of  the  second-formed  flood-plain,  and  this 
process  has  been  repeated  also  in  th*^  case  of  the  third 
terrace. 

In  the  normal  development  of  a  stream  after  the  stage  in 
a  certain  portion  of  its  course,  indicated  in  Fig.  ii,  is 
r'^ached,  the  stream  will  continue  to  deepen  its  channel,  and 
r  iy  cut  into  the  rock  below  the  flood-plain  deposits.  This 
stage  in  L^ie  process  is  illustrated  by  the  cro::s-section  shown 
below.  Should  subaerial  erosion  remove  the  alluvial  ma- 
terial indicated  by  dots  in  the  diagram,  a  rock  terrace  would 
be  left.     If  stream  development  progresses  and  a  second 


Fig.  12.     Ideal  Cross-Profile  of  a  Partially  AUtivial- Filled  Valley  Re-Excav&ted 
■  '  to  below  its  Ori^nal  Depth.  . 

approximation  to  baselevel  is  made,  all  of  the  alluvial  ma- 
terial and  a  portion  of  the  rocky  floor  on  which  it  rests  may 
be  removed. 

Other  ways  in  which  normal  alluvial  terraces  might  be 
formed  have  been  cited  by  Dodge.'  Suppose  that  a  stream 
whose  load  is  slightly  in  excess  of  its  carrying  power  ac- 
quires by  capture  the  head-waters  of  another  stream,  as  will 
be  considered  later.  In  the  district  thus  acquired  there 
^night  be  an  excess  of  carrying  power  over  load ;  if  such  was 
the  case,  the  capturing  stream  would  have  its  carrying  po'vcr 
increased   without   a  corresponding  increase  in  load,  and 

'  R.  E.  Dodge,  "  The  (Geographical  Development  of  Alluviul  Terraces,"  in 
boston  Socitty  of  Natural  History^  ProceeJings,  voi.  xxvi.,  p    263,  1894. 


11  r 
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m 


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11- 

in' 


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158 


RIVERS  OF  NORTH  AMERICA 


therefore  be  able  to  deepen  its  channel  in  previously  de- 
posited alluvium,  and  terri-ce  it. 

Again,  as  cited  by  Dodge,  a  stream  which  had  been  work- 
ing in  soft  rocks  might  cut  down  into  hard  rocks  underneath 
the  soft  ones.  The  effect  of  such  a  change  on  the  head- 
waters of  a  stream  would  be  to  decrease  its  load  and  enable 
it  to  corrade  in  its  alluvial  tract.  Hence,  without  varying 
in  volume  a  stream  might  be  able  to  terrace  an  alluvial  plain 
formed  while  it  was  previously  removing  soft  rock. 

Thus  in  several  ways,  or  as  a  result  of  the  combined  in- 
fluence of  two  or  three  normal  variations  in  streams,  alluvial 
terraces  might  result.  These  processes  of  terrace-making, 
however,  are  slow,  and  the  topographic  forms  resulting  may 
be  greatly  modified  or  even  obliterated  by  subaerial  denuda- 
tion as  fast  as  they  appear.  These  processes,  also,  are  a 
part  of  a  larger  process,  /.  c,  cutting  to  basclevel,  which  in- 
sures the  ultimate  destruction  of  the  topographic  features 
referred  to.  For  these  reasons  the  methods  of  terrace- 
making  just  considered  have  received  but  little  attention, 
and  their  results  are  difficult  to  recognise.  And,  besides, 
other  methods  of  terrace-forming  are  apt  to  produce  such 
conspicuous  results  that  the  terraces  due  to  what  has  been 
termed  the  normal  stream  development  are  usually  masked 
or  obliterated. 

Terraces  Due  to  Climatic  Changes. — In  considering  the 
various  influences  of  changes  of  climate  on  stream  deposi- 
tion, it  was  shown  that  heavy  rains  may  cause  the  tributaries 
of  a  stream  to  bring  to  the  main  channel  more  debris  than 
can  be  removed,  and  deposition  takes  place.  In  a  similar 
way  a  secular  change  of  climate  producing  an  increase  in 


STREAM   TERRACES 


159 


precipitation,  might  lead  to  the  filling,  especially  of  Icvv- 
grade  river-valleys,  and  the  raising  of  the  flood-plains 
throughout  all  of  their  lower  courses. 

A  climatic  change  which  would  admit  of  the  birth  and 
growth  of  glaciers  on  the  higher  portions  of  a  mountain 
range,  previously  deeply  stream-sculptured,  would  lead  to 
the  overloading  of  the  streams  below  the  glaciers  and  the 
thickening  and  broadening  of  the  flood-plains  throughout 
their  lower  courses. 

CHmatic  conditions  favourable  for  the  birth  and  growth 
of  glaciers  are  usually,  and  probably  always,  accompanied 
by  increased  precipitation  and  decreased  evaporation.  Thus 
for  several  reasons  the  occurrence  of  a  glacial  epoch  like 
that  in  late  geological  time,  when  one-half  of  North  America 
was  occupied  by  ice-sheets,  would  favour  the  filling  of  pre- 
existing valleys  with  debris.  When  the  climate  experienced 
a  reverse  change  and  the  glaciers  melted,  the  draining 
streams  would  for  a  time  be  still  more  deeply  flooded,  and 
additional  quantities  of  debris  carried  ;rom  high  to  low 
regions.  If  a  warmer  and  drier  climate  should  succeed  a 
glacial  epoch,  the  streams,  no  longer  heavily  loaded,  would 
begin  the  task  of  removing  the  debris  deposited  in  their 
valleys  during  the  preceding  time  of  overloading.  As  the 
streams  deepened  their  channels  h\  the  alluvium  previously 
deposited,  portions  of  the  flood-plains  left  intact  would  ap- 
pear as  terraces,  and  the  elevation  of  their  surfaces  would 
record  the  depth  to  which  the  valleys  had  been  filled  with 
debris. 

This  process  of  removing  the  accumulations  of  debris 
clogging  a  valley  might  be  accompanied  by  the  formation  of 


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RIVERS  OF  NORTH  AMERICA 


terraces  at  lower  levels,  according  to  the  laws,  cited  above, 
governing  the  normal  development  of  streams.  As  will  be 
shown  later,  however,  a  still  more  potent  agency  in  the 
formation  of  the  lower  terraces  would  be  climatic  changes 
and  periodic  elevation  of  the  land. 

Should  several  glacial  stages  occur  with  intervals  of  milder 
and  less  humid  climatic  conditions  intervening,  it  is  evident 
that  the  terraces  resulting  from  the  trenching  of  the  first- 
formed  flood-plains  might  be  obliterated  by  subsequent  de- 
position, and  the  surface  of  the  debris  in  the  valleys  be  carried 
higher  than  during  the  first  ice  invasion;  or  the  valleys,  cut 
in  the  first-formed  flood-plain,  might  be  only  partially  re- 
filled, and  when  excavation  was  renewed,  lower  terraces 
would  be  formed.  :'   ^ 

The  conclusion  that  glacial  conditions  would  lead  to 
the  filling  of  pre-existing  valleys  downstream  from  alpine 
glaciers  or  about  the  margins  of  piedmonts  and  continental 
ice-sheets,  and  portions  of  these  deposits  be  left  as  terraces 
when  corrasion  was  resumed,  is  sustained  by  an  abundance 
of  examples  throughout  the  northern  portion  of  the  United 
States  and  Canada.  In  the  valleys  in  this  region  terraces 
excavated  in  material  deposits  by  glacial  streams  are  mag- 
nificently displayed.  On  the  head-waters  of  Columbia  River 
in  Washington  and  Idaho,  terraces  of  the  nature  here  con- 
sidered are  perhaps  as  well  developed,  and  their  history  as 
easily  read,  as  in  any  other  portion  of  the  continent.  The 
great  canyon  of  Snake  River,  the  principal  tributary  of  the 
Columbia,  was  excavated  to  its  present  depth, — four  thou- 
sand feet  throughout  a  considerable  portion  cf  its  course, — 
previous  to  the  Glacial  epoch.     During  that  epoch,  glaciers 


1  above, 
;  will  be 

in  the 
changes 

f  milder 
evident 
he  first- 
nent  de- 

2  carried 
leys,  cut 
ially  re- 
terraces 

lead   to 

1  alpine 

tinental 

terraces 

ndance 

United 

[terraces 

|re  mag- 

ia  River 

:re  con- 

^tory  as 

The 

of  the 
Ir  thou- 
|urse, — 

[laciers 


STREAM    TERRACES 


i6i 


existed  in  the  more  elevated  valleys  and  about  the  sum- 
mits of  the  mountains  of  Idaho.  The  branches  of  the 
Snake  were  flooded,  and  brought  such  quantities  of  debris 
to  the  canyon  of  the  main  stream  that  throughout  hun- 
dreds of  miles  of  its  course  it  became  filled  to  a  depth  of 
three  hundred  and  sixty  feet.  When  the  glaciers  passed 
away  and  the  streams  were  no  longer  supplied  with  debris 
by  them,  and  still  more  effectually  when  a  mild  and  but 
moderately  humid  climate  prevailed,  the  streams  were 
enabled  to  attack  their  flood-plains  and  cut  valleys  and 
canyor.s  through  them.  Snake  River  has  now  removed  by 
far  the  greater  portion  of  the  coarse  gravel  and  boulders 
that  formerly  occupied  its  canyon,  and  has  resumed  the  task 
of  deepening  its  channel  in  the  hard  rock  beneath.  Episodes 
similar  to  that  just  referred  to  in  the  history  of  Snake  River, 
but  with  various  minor  modifications,  occurred  in  the  lives  of 
tens  of  thousands  of  streams  not  only  in  the  northern  part 
of  North  America,  but  as  far  south  as  the  Gulf  of  Mexico 
and  also  in  the  Rocky  Mountains  and  Sierra  Nevada,  as  a 
lesult  of  the  climatic  change  to  which  the  Glacial  epoch  was 
due. 

In  studying  the  effects  of  changes  in  climate  on  the  be- 
haviour of  streams,  the  fact  should  be  borne  in  mind  that 
such  changes,  although  by  reason  of  the  comparatively  brief 
time  during  which  man  has  taken  account  of  secular  vari- 
ations in  atmospheric  phenomena  they  are  commonly 
considered  as  exceedingly  slow  in  their  occurrence  and  cm- 
bracing  but  a  moderate  range,  appear  relatively  rapid  and 
of  well-marked  amplitude  when  such  periods  of  time  as  are 
involved  in  geographic  cycles  are  studied.     Many  of  our 


1. 


^ 


I':: 


4 
I 


W*  \m' 


s 


162 


RIVERS  OF  NORTH  AMERICA 


rivers,  as,  for  example,  the  Susquehanna,  Mississippi,  and 
Columbia,  were  far  advanced  in  their  development  before 
the  beginning  of  the  Glacial  epoch.  The  time  that  has 
elapsed  since  the  melting  of  the  continental  glaciers  on  the 
head-waters  of  these  rivers  is  but  a  small  fraction  of  the  cur- 
rent geographic  cycles.  Many  annual  climatic  changes,  as 
is  well  known  to  everyone,  occur  while  even  a  meadow 
broo'-  undergoes  but  slight  modifications ;  in  a  similar  way, 
as  is  well  known  to  geographers,  many  secular  changes  in  cli- 
matic conditions  may  take  place  during  the  life  history  of  a 
great  river. 

Terraces  Due  to  Elevation  of  the  Land. — The  manner  in 
which  a  stream  carries  on  its  work,  it  will  be  remembered, 
is  controlled  in  an  important  way  by  declivity.  Conse- 
quently, changes  in  the  elevation  of  the  land  must  have  a 
direct  bearing  on  the  history  of  the  streams  draining  an 
area  thus  affected.  The  movements  in  the  earth's  crust 
referred  to  are  known  to  have  modified  the  surface  slopes 
throughout  large  areas,  and  frequently  to  be  of  the  nature 
of  a  tilting  of  the  land.  Other  movements  occur,  but  at 
present  let  us  consider  simply  the  effects  of  the  tilting  of  a 
region  drained  by  a  large  river  on  the  problem  of  terrace- 
making.  ^ 

A  tilting  of  the  rocks  which  decreases  the  gradient,  and 
consequently  the  velocity,  of  a  stream,  other  conditions  re- 
maining the  same,  will  favour  deposition,  and  may  lead  to 
the  partial  or  complete  filling  of  its  previously  formed  valley. 
The  flood-plain  deposits  would  then  increase  in  thickness 
and  become  broader  at  the  surface.  In  other  words,  a  de- 
crease in  velocity  favours  the  process  of  aggrading. 


STREAM    TERRACES 


163 


If  the  region  drained  by  the  Connecticut,  for  example, 
be  considered  as  a  plane  gently  inclined  southward,  and  to 
be  affected  by  a  movement  in  the  earth's  crust  which  de- 
creases the  gradient  of  the  river,  the  depression  of  the  land 
being  least  at  the  south  and  progressively  increasing  north- 
ward,— that  is,  the  hinge-line,  so  to  speak,  on  which  the 
tilted  block  of  the  earth's  crust  moves,  being  situated  near 
its  southern  margin, — the  main  trunk  of  the  river  would  have 
its  current  slackened,  and  its  transporting  power  diminished, 
while  the  gradients  of  the  branches  of  the  river  coming  in 
from  the  east  or  west  would  be  but  slightly  affected.  The 
direct  result  of  such  a  change  would  be  to  favour  deposition 
in  the  main  valley  and  to  a  less  extent  in  its  branches. 

If,  after  such  a  change  of  grade  as  has  been  postulated, 
when  the  valley  of  the  Connecticut  has  become  deeply  filled 
and  a  broad  flood-plain  spread  out,  we  imagine  the  land  to 
remain  stationary,  the  branches  of  the  main  river  would  cut 
down  their  channels,  thus  decreasing  their  velocities  and 
diminishing  the  amount  of  debris  carried  annually  to  the 
main  valley.  When  this  stage  had  been  reached,  the  Con- 
necticut would  begin  to  cut  a  channel  through  its  previously 
formed  flood-plain,  as  in  the  case  of  normal  stream  develop- 
ment already  considered. 

In  case  the  inclined  plane  drained  by  the  Connecti- 
cut should  experience  a  reverse  movement  after  its  valley 
had  become  deeply  filled — that  is,  if  elevation  should 
occur,  the  hinge-line  retaining  its  former  position, — the 
gradient  of  the  main  stream  and  of  all  its  branches  flowing 
southward  would  be  increased,  while  the  lateral  branches 
would  be  but  little  affected.     The  increased  gradient  of  the 


164 


HI  VERS  OF  NORTH  AMERICA 


'i     m 


main  stream  would  give  its  waters  greater  velocity,  thus 
favouring  corrasion  at  the  expense  of  deposition,  and  a 
channel  would  be  cut  through  the  previously  formed  flood- 
plain.  Portions  of  the  flood-plain  not  removed  would  re- 
main as  terraces.  Imagine  the  re-elevation  at  the  northern 
border  of  the  tilted  area  to  be  one  hundred  feet,  and  to  de- 
crease to  zero  at  the  hinge-line  at  the  south.  The  result 
would  be  acceleration  of  velocity  in  the  extreme  head 
branches  flowing  southward  ;  this  might  cause  them  to  bring 
more  debris  to  the  main  stream  than  it  could  transport,  but 
the  branches  from  the  east  and  west  being  but  slightly 
affected,  the  more  probable  result  would  be  the  deepening 
of  the  bed  of  the  main  stream  throughout.  The  river 
would  excavate  a  channel  through  its  previously  formed 
flood-plain,  leaving  portions  of  it  on  either  side  of  the  valley 
as  terraces.  When  the  river,  after  adjusting  itself  to  the 
new  conditions,  began  to  broaden  its  channel  and  spread 
out  a  second  flood-plain,  it  would  be  flowing  a  hundred  feet 
below  its  former  bed  in  the  upper  portion  of  its  course,  but 
this  difference  would  gradually  decrease  downstream  and 
become  zero  where  the  hinge-line  was  crossed.  The  south- 
ward or  down-stream  slope  of  the  surface  of  the  old  flood- 
plain,  portions  of  which  remain  for  a  time  as  terraces,  would 
therefore  be  greater  than  the  slope  or  gradient  of  the 
readjusted  stream.  This  postulated  case  thus  furnishes  an 
explanation  of  the  fact  that  when  a  number  of  stream  ter- 
races occur  on  the  border  of  a  valley,  they  are  not  only  not 
horizontal,  but  have  different  gradients.  The  gradient  of 
every  stream  terrace  is  determined  by  the  gradient  of  the 
parent  stream  at  the  time  it  was  formed. 


if  ;;!-■ 


STREAM    TERRACES 


165 


The  terraces  originating  in  the  several  ways  thus  far  con- 
sidered consist  of  alluvium,  which  was  deposited  in  a  pre- 
viously formed  river  valley,  and  the  surface  of  each  terrace 
is  a  portion  of  a  flood-plain.  In  cross-section,  such  terraces 
would  have  the  characteristics  shown  by  the  diagram  on 
page  156,  introduced  in  connection  with  the  discussion  of 
what  are  termed  normal  te^'-aces,  and  would  be  cut  through 
or  finally  removed  during  subsequent  stream  development 
in  the  manner  already  described,  unless  subsidence  carried 
them  below  baselevel.  In  the  discussion  just  presented,  we 
have  assumed  a  river  valley  to  have  been  deeply  filled  with 
alluvium  previous  to  the  elevation  of  the  land  which  enabled 
the  stream  to  deepen  its  channel.     This  assumption  is  not 


Fig.  13.     Ideal  Cross-Section  of  a  Valley  with  Terraces  Cut  in  Solid  Rock  and 

Covered  with  Alluvium, 

necessary,  however,  and  numerous  instances  might  be  cited 
where  terraces  in  solid  rock  have  resulted  from  accelerated 
corrasion  due  to  periodic  uplifts.  Imagine  a  stream  like  the 
Connecticut  to  have  broadened  its  valley  and  spread  out  a 
flood-plain,  and  then  an  elevation  to  take  place  as  before. 
Accelerated  velocity  may  enable  the  stream  to  lower  its  bed 
so  as  to  cut  through  the  flood-plain  deposits  and  into  the 
rocks  beneath.  A  broadening  of  the  new  channel  may  then 
occur,  and  renewed  elevation  allow  the  process  to  be  re- 
peated. With  each  upheaval  the  stream  cuts  deeper  into 
the  rocks,  leaving  each  time  a  terrace  of  solid  rock  with  a 
sheet  of  alluvium  on  its  surface.     The  characteristic  features 


*fl!! 

t! 


!i 


I 


III  ! 


J 

.1  l'i|,M 

i.;,:|i 

166 


RIVERS  OF  NORTH  AMERICA 


of  a  cross-section  of  such  a  terraced  valley  are  shown  in 
the  ideal  diagram,  Fig.  13.  As  the  excavation  of  solid  rock 
is  normally  a  slow  process,  the  sheet  of  alluvium  covering 
the  terraces  would  be  apt  to  be  removed  by  rain,  rills,  etc., 
and  rock  terraces  but  scantily  covered  or  without  debris 
be  exposed. 

The  formation  of  terraces  during  what  has  been  termed 
the  normal  development  of  a  stream — that  is,  when  changes 
of  level  have  not  occurred,  and  climatic  variations,  etc.,  have 
not  materially  affected  its  volume,  velocity,  or  load — is  an 
extremely  slow  process,  and,  as  previously  stated,  it  is  prob- 
able that  atmospheric  agencies  under  most  climatic  condi- 
tions would  destroy  the  terraces  as  fast  as  formed.  For  this 
and  other  reasons  it  is  believed  that  most  of  the  terraces  on 
the  borders  of  stream-cut  valleys  are  records  of  climatic 
changes  which  caused  excessive  deposition  in  low-grade 
valleys  followed  by  a  period  of  erosion ;  or  are  due  to  land 
oscillation. 

Bottom  Terraces. — Still  another  variety  of  terraces  is 
formed  by  streams  by  deposition  when  their  bottom  loads 
are  small.  When  their  bottom  currents  are  underloaded,  as 
we  may  term  the  condition  here  referred  to,  the  material  is 
carried  forward  like  a  wave,  in  the  manner  in  which  a  ripple 
in  sand  is  produced  under  the  influence  of  a  wind-  or  water- 
current,  and  deposited  with  a  steep  escarpment  facing  down- 
stream. These  bottom  terraces  have  broad,  gently  ascending 
surfaces  in  the  direction  of  the  flow  of  the  current,  and 
steep  escarpments,  facing  downstream,  and  trend  in  general 
at  right  angles  to  the  flow  of  the  water,  but  are  usually 
lobed   on    their   lower   margins.     Such   terraces   or   broad 


STREAM    TERRACES 


167 


ripples  may  be  seen  in  process  of  growth  in  many  clear 
streams  which  have  moderate  bottom-loads  of  coarse  sand 
and  gravel.  They  are  frequently  several  feet  or  even  yards 
broad,  with  escarpments  from  a  few  inches  to  a  few  feet 
high.  Although  of  minor  importance  when  considered  in 
connection  with  associated  stream-made  topographic  forms, 
yet  under  special  conditions,  as  when  a  broad  stream  is  mov- 
ing debris  over  a  gentle  slope,  they  might  become  relatively 
conspicuous  if  the  stream  should  be  diverted.  There  is  a 
gradation  between  bottom  terraces  of  the  nature  just  con- 
sidered and  delta  terraces  which  would  repay  investigation. 

Delta  Terraces  and  Current  Terraces.  -In  the  discussion 
of  deltas  in  a  previous  chapter,  it  was  shown  that  they  are 
formed  where  streams  deposit  their  loads  on  entering  still 
water.  Now,  streams  sometimes  expand  and  have  sluggish 
currents  so  as  to  simulate  lakes.  When  this  happens  a 
tributary  stream  freighted  with  debris  may  drop  a  por- 
tion of  its  load  and  build  up  a  delta-like  deposit  at  its 
mouth.        •-.,:■.:/'■-■'-"-'::,■:".„  -v  ,,';">■■,-;  ^^   -...-,-■,.,'■-,■ 

The  most  favourable  conditions  for  this  process  are  when 
low-grade,  sluggish  rivers  extend  into  embayments  on  their 
borders,  as  the  mouths  of  tributary  valleys,  and  a  stream 
from  the  tributary  valley  brings  in  sediment.  A  lowering 
of  the  main  stream  after  such  a  delta  has  been  formed 
would  leave  it  as  a  terrace.  Such  structures  have  been 
termed  delta  terraces  by  Edward  Hitchcock.'  A  section  of 
a  delta  terrace  would  reveal  a  series  of  inclined  beds,  as 
shown  in  the  diagram  oh  page  126,  and  possibly  the  upper 

'  "  Illustrations  of  the  Earth's  Sii'^face."  Smithsonian  Contributions  to  Know- 
ledge, vol.  ix.,  pp.  32-34.  1857. 


1 68 


RIVE  us  OF  NORTH  AMERICA 


''i';i 


and  lower  members  of  a  typical  delta  built  by  a  high-grade 
stream  as  well.  The  surface  of  such  a  delta  terrace  would 
have  a  slope  corresponding  with  the  grade  of  the  supplying 

stream. 

-if'' 

The  current  of  a  river  washes  its  banks  in  much  the  same 
way  as  the  currents  in  lakes  wash  their  shores.  The  study 
of  the  action  of  lake  cu«"rents  has  shown  that  they  bear 
along  debris,  and  drop  it  in  part  so  as  to  form  what  are 
known  as  built  terraces.  The  current,  especially  of  a  broad 
river,  behaves  in  much  the  same  manner.  Debris  brought 
by  tributary  streams,  or  derived  from  localities  where  the 
river  is  corrading  its  banks,  is  carried  down  stream  and 
may  be  deposited  adjacent  to  the  shore,  so  as  to  form  a 
built  terrace.  A  subsidence  of  the  waters  would  leave  the 
terrace  exposed.  Its  surface  would  slope  gently  toward  the 
stream,  and,  as  in  the  case  of  all  river  terraces,  would  have 
a  gradient,  when  followed  along  the  valley  on  the  side  of 
v/hich  it  was  formed,  corresponding  with  the  surface  gradient 
of  the  building  stream.  In  cross-section  such  a  terrace 
would  reveal  the  structure  characteristic  of  built  lake- 
terraces,  the  general  features  of  which  are  shown  in  the 
following  ideal  diagram.     The   slope   rising  above  such  a 


f 

t 

..h 

■i 

i 

id 

; 

Fig.  14.     Ideal  Cross-Section  of  a  Cyrrent-Built  Terrace. 

terrace  may  be  the  valley  side,  produced  by  stream  corrasion 
and  weathering,  or  be  a  steeper  slope  due  to  lateral  corra- 


STREAM    TERRACES 


169 


sion  of  the  current  and  correspond   more  nearly  with  the 
"  sea-cliff  "  above  a  lake  terrace. 

The  waves  and  currents  of  a  broad  river  may  lead  to  cor- 
rasion  along  its  shores  in  the  same  manner  as  in  a  lake,  and 
cut  terraces  result.  A  miniature  example  of  this  is  shown 
in  Fig.  F,  Plate  II. 

So  far  as  the  present  knowledge  of  stream  terraces  allows 
one  to  judge,  it  does  not  appear  that  those  built  after  the 
manner  of  lake  terraces,  as  just  described,  are  common.  In 
fact,  delta  terraces  and  current  terraces,  as  they  may  be 
termed,  depend  for  their  origin  on  a  delicate  balancing  of 
conditions  which  apparently  is  seldom  reached. 

Delta  terraces  and  current  terraces  formed  on  the  sides  of 
streams  are  of  interest,  as  they  constitute  a  group,  although 
small  and  of  minor  importance,  which  may  be  designated  as 
built  terraces  in  distinction  from  other  stream  terraces  which 
are  due  to  both  deposition  and  excavation,  or  to  excavation 
alone.  The  downward  slope  bordering  the  nearly  flat  sur- 
face of  a  built  terrace  is  due  to  deposition ;  in  the  other 
varieties,  this  slope  is  produced  by  excavation. 

Glacial  Terraces. — The  terraces  built  by  streams  con- 
jointly with  glaciers  need  not  claim  much  attention  at  this 
time,  since  they  derive  their  greatest  interest  from  their 
connection  with  the  ice-bodies  about  which  they  are  formed. 
When  a  glacier,  however,  or  perhaps  more  frequently  a  3tag- 
nnnt  ice-mass,  occupies  a  valley,  streams  sometimes  bring 
gravel,  sand,  etc.,  and  deposit  it  along  the  margin  of  the  ice 
so  as  to  give  a  level  floor  to  the  space  intervening  between 
the  ice  and  the  valley  border.  After  this  space  has  been  filled 
to  a  greater  or  less  depth  and  the  ice  melts,  the  deposit  re- 


4^ 


1    ;.     ---■»^ 

ii 

DMIBI 


It' 


f 


I/O 


RIVERA  OF  NORTH  AMERICA 


mains  as  a  terrace.'  Such  terraces  have  approximately  level 
surfaces,  are  composed  of  current-bedded  gravel  and  sand, 
and  perhaps  certain  occasional  boulders  or  angular  rock- 
ma«5ces,  but  do  not  exhibit  the  arrangement  of  coarse  and 
fine  material  characteristic  of  flood-plains;  and,  besides, 
th^ir  down-stream  gradients  are  markedly  different  from 
those  of  true  stream-terraces. 

Relative  Age  of  Terraces. — When  stream  terraces  occur 
one  above  another  on  the  side  of  a  valley,  the  highest  in 
the  series  is  usually  the  oldest.  But  exceptions  to  this 
rule  may  occur,  as  when  changes  of  level  lead  to  the  build- 
ing of  delta  or  current  terraces  on  the  surface  of  previously 
formed  terraces.  Again,  a  valley  in  which  a  terrace  has 
been  cut  in  solid  rock  might  become  filled  with  alluvium 
so  as  to  bury  the  terrace,  and  re-excavation  again  bring  it  to 
light,  and  form  another  terrace  at  a  higher  level;  the  lower 
ter-ace  would  then  be  older  than  the  one  above  it. 

In  a  series  of  alluvial  terraces  in  which  the  highest  is  the 
oldest,  each  one  or  each  pair,  if  fragments  of  the  same  flood- 
plain  are  left  on  each  side  of  the  valley,  is  a  remnant  of 
d  flood-plain,  and  the  material  in  the  highest  terrace  is 
younger  than  the  main  portion  of  each  lower  ter-ace ;  but  the 
surface  portion  of  each  terrace  was  worked  over  and  re- 
distributed at  the  time  the  flood-plain  of  which  it  is  a  part 
Was  formed,  and  hence  may  be  said  to  be  younger  than  the 
material  in  each  higher  terrace. 

'The  terraces  here  referred  to  have  been  termed  "  kaine  terraces"  oy  R. 
L.  Salisbury,  Gtological  Survey  of  Nfw  Jersey^  Annual  Report  for  i8gj,  ^. 
'55i  '56.  Similar  topographic  forms  were  previously  termed  "moraine  ter- 
races" by  G.  K.  Gilbert,  U.  S.  Geological  Survey,  Monographs,  vol.  i.,  p.  81, 
1890. 


STREAM   TERRACES 


171 


Other  Terraces. — Terraces  similar  to  those  formed  by 
streams  originate  in  other  ways,  and  it  is  important  that 
the  student  of  geography  and  geology  should  be  able  to  dis- 
tinguish those  which  owe  their  origin  to  one  series  of  agencies 
from  those  belonging  to  oLher  categories. 

Cut  terraces  in  rock  or  in  loose  material  are  a  characteris- 
tic feature  of  lake  and  ocean  shores,  as  are  also  delta  and 
current  terraces.  In  nearly  all  of  their  main  features,  these- 
terraces  are  similar  to  stream  terraces  except  that  they  are 
essentially  horizontal  when  traced  in  the  direction  of  their 
length.  Movements  in  the  earth's  crust,  however,  may  tilt 
a  previousl)^  horizontal  terrace  so  as  to  give  it  a  gradient 
closely  approximating  to  the  normal  slope  of  a  stream 
terrace.  A  similar  tilting  of  the  land  might  affect  a  river 
terrace  so  as  to  alter  its  gradient  and  perhaps  make  it  hori- 
zontal. In  cases  of  this  sort  associated  topographic  features 
would  usually  furnish  the  best  clue  to  the  true  history. 
River  terraces  are  formed  in  comparatively  narrow  valleys, 
while  lakes  may  occupy  valleys  of  any  shape.  Lake  ter- 
races are  usually  accompanied  by  escarpments  which  rise 
above  them,  termed  "  sea-cliffs  "  ;  these  may  or  may  not  be 
characteristically  different  from  the  corresponding  slopes 
above  river  terraces.  The  normal  lake-terrace  is  either  cut- 
and-built — that  is,  it  is  a  shelf  mrde  by  excavation  with  a 
current-built  covering  on  its  surface, — or  may  be  entirely 
a  deposit  formed  by  construction.  Stream  terraces  do  not 
usually  have  this  structure,  but  yet  may  have  it.  As  may  be 
judged,  the  tests  just  suggested  might  not  lead  to  definite 
conclusions.  In  fact,  in  the  case  of  abandoned  stream-  and 
lakc-tcrraces,  that  is,  when  a  former  lake  basin  has  been 


Hi 


SSSSSumm 


t]i 


■t   li 


172 


RIVERS  OF  NORTH  AMERICA 


emptied  and  perhaps  has  a  stream  flowing  through  it,  or 
when  a  former  stream  valley  is  no  longer  a  line  of  drainage, 
it  is  frequently  difficult  to  satisfactorily  determine  their 
origin.  In  such  an  instance,  if  the  former  topcj^raphy  is 
rot  greatly  altered,  it  may  be  possible  to  work  out  the 
history  of  the  changes  that  have  occurred  and  to  construct 
a  map  of  the  country  as  it  existed  when  the  terraces  were 
formed,  and  thus  be  able  to  decide  whether  flowing  water 
or  bodies  of  still  water  were  respo.isible  for  the  terraces. 

River  terraces  frequently  make  a  direct  connection  with 
lake  terraces  so  that  the  place  of  junction  may  be  difficult 
to  determine.  The  main  difference  to  be  looked  for  in  such 
an  instance  would  be  a  change  from  horizontali^^y  to  an  in- 
clination in  the  surfaces  of  the  terraces  where  it  passed  from 
the  lake  valley  into  the  stream  valley.  In  a  tilted  or  other- 
wise disturbed  region,  where  both  lake  and  stream  terraces 
occur,  A  difference  in  their  gradients  may  still  be  recognis- 
able, and  assist  in  their  discrimination. 

Instances  occur,  however,  in  disturbed  and  eroded  re- 
gions, where  only  fragments  of  terraces  remain,  when  it 
is  practically  impossible  to  tell  whether  they  record  lacustral 
or  stream  conditions.  River  terraces  also  pass  into  terraces 
made  about  the  borders  of  estuaries  and  on  ocean  shores. 
Here,  again,  when  disturbances  occur  and  the  estuaries  a»e 
emptied  of  their  water  and  the  streams  have  been  diverted, 
difficulties  in  interpreting  the  records  might  arise.  The 
sedimentary  deposits  made  on  tho  floor  of  the  estuaries  and 
the  evidences  of  life  buried  in  them,  as  well  as  the  fossils  in 
the  terraces  themselves,  might  here  furnish  assistance.  The 
shells  in  river  terraces  will  be  fresh-water  or  land  species; 


STREAM    TERRACES 


173 


id  re- 
icn   it 
lustral 
Irraccs 
liores. 
:s  are 
:rted, 
The 
IS  and 
iils  in 
The 
jcies ; 


while  those  in  the  estuary  sediment  and  terraces  will  be,  in 
part  at  least,  such  as  inhabit  brackish  or  saline  water. 

Terraces  also  result  from  the  weathejing  of  the  outcrops 
of  alternating  hard  and  soft  strata.  When  the  strata  are 
horizontal  or  but  slightly  inclined,  the  hard  beds  may  stand 
out  as  shelves  or  terraces,  as,  for  example,  in  the  sides  of  a 
valley  cut  in  stratified  rocks.  The  downward  slope  of  such 
a  terrace  is  the  exposed  edges  of  hard  layers,  and  may  be 
steep  or  gentle  according  to  climatic  and  other  conditions; 
the  slope  rising  above  the  terrace  is  formed  of  the  edge  of 
the  weak  strata  above  the  terrace-making  layer,  and  is 
usually  a  gentle  slope  unless  the  layer  is  thin  and  another 
hard  terrace-making  layer  occurs  just  above.  The  arrange- 
ment of  resistant  and  weak  strata  in  the  case  of  these 
terraces  of  differential  erosion  usually  makes  it  easy  to  dis- 
tinguish them  from  river  terraces.  There  is  an  absence, 
also,  on  the  terraces  of  this  nature,  of  stream  deposits, 
and  this  negative  evidence  might  assist  in  the  diagnosis. 
Weathered  debris  falling  on  the  surface  of  a  terrace  of 
differential  erosion  may  simulate  stream  deposits,  however, 
and  lead  to  erroneous  conclusions. 

Fractures  in  the  rocks  aiorg  whicli  differential  movement 
of  the  sides  has  taken  place,  producing  what  are  known  as 
faults,  may  also  give  origin  to  topographic  forms  of  a  terrace- 
like character,  but  these  are  usually  irregular,  with  reference 
to  both  horizontal  and  verticil  planes,  and  in  most  instances 
are  easily  distinguishable  from  stream  or  other  terraces.   -^ 

Landslides  also  produce  terrace-like  forms,  but  these  are 
seldom  continuous  for  considerable  distances,  and  are  usually 
so  irregular  and  bear  such  relations  to  the  slopes  from  which 


i 


'74 


RIVERS  OF  NORTH  AMERICA 


the  fallen  blocks  descended,  that  their  origin  may  usually 
be  readily  determined.  When  a  landslide  occurs  it  fre- 
quently happens  that  the  displaced  material  acquires  a  sur- 
face slope  towaid  the  place  from  which  it  came.  This 
backward  slope  frequently  produces  basins  in  which  lakes 
and  swamps  occur,  thus  furnishing  additional  evidences 
bearing  on  the  origin  of  the  terrace-like  forms  produced.' 

Terraces  due  to  still  other  causes  might  be  enumerated 
and  the  means  for  their  discrimination  indicated,  but  I  be- 
lieve those  most  nearly  simulating  stream  terraces  have  been 
referred  to.  '^he  reader  who  may  desire  to  follow  this  sub- 
ject farther  will  find  assistance  in  the  treatises  named  below.' 

General  Distribution  of  Stream  Terraces. — In  North 
America  stream  terraces  may  be  said  to  occur  on  the 
borders  of  nearly  every  river  valley  north  of  the  central 
part  of  the  United  States,  but  arc  less  conspicuous  in  the 
more  south-eastern  States  and  about  the  Gulf  of  Mexico: 
the  reason  being  that  the  northern  half  of  the  continent 
was  occupied  by  glaciers  in  late  geological  time,  and  has 
also  undergone  movements  of  the  nature  of  elevation  and 
depression  throughout  broad  areas;  while  in  the  southern 
half  of  the  continent  there  are  no  records  of  glaciation  ex- 
cept on  high  mountains  in  the  south-west,  and  evidences  of 
recent  changes  of  level  are  seldom  pronounced. 

Stream  terraces  extend  far  southward  from  the  formerly 
glaciated  region,  for  the  reason  that  the  glaciers  drained  by 

'  1.  C.  Russell,  "Topographic  Changes  Due  to  Landslides,"  Popular  Scienct 
Monthly,  vol.  liii.,  i^igS,  in  press. 

*  G.  K.  Gilbert,  U.  S.  Geolot^ical  Survfv,  Mot,  ^z^aphs,  vol.  i.,  pp.  78-86,  i8(K>. 
W  J  McGee,  U.  S.  Geologital  Purvey,  nth  Annual  Riport,  part  i..  pp.  256- 
273,  1889-90. 


STREAM    TERRACES 


175 


southward-flowing  streams  furnished  more  debris  than  the 
streams  could  remove,  and  they  became  overloaded  and  con- 
sequently filled  in  their  previously  excavated  valleys.  When 
the  glaciers  disappea*"ed  and  the  streams  were  no  longer  over- 
loaded, they  cut  channels  through  their  previously  formed 
flood-plains,  and  left  portions  of  them  as  terraces  on  their 
sides.  This  T^ro'-ess  was  aided  also  by  a  general  depression 
at  the  north,  cue  to  some  extent,  it  is  believed,  to  the 
weight  of  the  ice,  and  in  part  to  the  effect  of  the  lowering 
of  the  temperature  to  a  considerable  depth  in  the  earth's 
crust  and  a  consequent  contraction  and  depression  of  the 
surface,  and  a  partial  re-elevation  when  the  glaciers  vanished. 

Many  of  the  beautiful  river-valleys  of  New  England  owe 
much  of  their  attractiveness  to  the  gracefully  bending  curves 
traced  on  their  borders.  Numerous  towns  and  villages  in 
that  region  are  indebted  for  their  sightly  locations  to  the 
terraces  on  which  they  are  built.  Present  flood-plains  and 
abandoned  portions  of  former  flood-plains  afford  rich  agricul- 
tural lands.  These  were  among  the  first  areas  to  be  cleared 
and  cultivated  after  European  immigration  began.  A 
direct  relation  between  the  effects  of  distant  and  far-reach- 
ing changes  in  geography  and  the  advance  and  growth  of 
civilisation  is  here  abundantly  illustrated 

What  has  been  said  of  the  terraced  valleys  of  New  Eng- 
land is  true  in  varying  degrees  of  a  great  area  to  the  north 
embraced  in  the  south-eastern  provinces  of  Canada,  and  of  a 
still  broader  region  to  the  west  including  New  York,  Penn- 
sylvania, Ohio,  and  thence  north-westward  to  the  Pacific. 

The  valleys  of  the  Ohio  and  of  many  of  its  tributaries  are 
noted  for  their  terraces.     In  this  region  and  also  in  the  val- 


176 


RIVERS  OF  NORTH  AMERICA 


leys  tributary  to  the  upper  Mississippi,  the  numerous  terraces 
are  due  principally  to  the  re-excavation  of  pre-glacial  val- 
leys, in  which  overloaded  glacial  streams  dropped  the 
burdens  too  heavy  for  them  to  carry.  The  effects  of  the 
changes  in  drainage  accompanying  the  great  ice  invasion  at 
the  north  may  be  traced  throughout  the  length  of  the  Mis- 
sissippi, but  become  less  and  less  conspicuous  towards  its 
mouth. 

The  borders  of  the  streams  flowing  to  the  Gulf  of  Mexico 
{other  than  the  Mississippi),  including  the  larger  rivers  of 
the  Texas  region  and  those  draining  the  eastern  slope  of  th^ 
Appalachians  south  of  Maryland,  are  mostly  without  con- 
spicuous terraces.  The  streams  in  this  region  did  not  feel 
the  direct  influence  of  the  glaciers  at  the  north  and  have 
passed  their  period  of  youth,  except  on  their  extreme  head- 
waters. The  terraces  they  may  have  formed  as  a  part  of 
their  normal  aggrading  and  re-excavation  have  been  re- 
moved principally  by  weathering,  and  the  comparatively 
gentle  slopes  of  their  valleys  show  vertical  scorings  due  to 
the  action  of  rills  and  not  the  nearly  horizontal  lines  which 
record  higher  water  stages.  Such  terraces  as  do  occur 
along  the  sides  of  southern  rivers  are  in  part  remnants  of 
ancient  baselevel  plains,  or,  in  some  instances,  the  result 
of  local  changes  in  elevation.  These  statements  are  inten- 
tionally made  general,  as  it  is  only  the  more  marked  differ- 
<;nces  between  the  characteristics  of  southern  and  northern 
valleys  to  which  attention  is  here  sought  to  be  directed. 

The  rivers  flowing  eastward  from  the  Rocky  Mountains, 
like  the  Platte,  Missouri,  Arkansas,  etc.,  are  bordered  by 
terraced  slopes  in  a  portion  of  their  valley  tracts,  and  for 


STREAM    TERRACES 


177 


many  miles  in  the  plains  tracts  after  le?  .ing  the  mountains, 
but  well  out  on  the  Great  Plains  they  are  in  part,  and  per- 
haps i.Tiostly,  engaged  at  the  present  time  in  aggrading 
previously  formed  valleys,  and  terraces  are  not  conspicuous. 

One  reason  for  the  presence  of  conspicuous  terraces  adja- 
cent to  the  mountains  and  in  the  valleys  'eroded  in  them 
previous  to  the  great  extension  of  the  glaciers,  is  that  the 
streams  dropped  the  coarser  and  heavier  portions  of  their 
loads  at  those  localities,  and  bore  on  only  fine  material  to 
their  low-grade  plains  tracts.  Subsequently,  when  the  flood- 
plains  thus  formed  were  terraced,  the  escarpments  in  coarse 
and  in  part  cemented  gravel  and  boulders  retained  their 
slopes  for  a  longer  period  than  the  similar  escarpments  in 
fine  material  a  hundred  miles  or  more  downstream.  In 
addition  to  this,  it  is  to  be  noted  that  the  gradients  of  the 
highest  terraces  are  greater  than  the  gradients  of  the  streams 
as  they  flow  at  the  present  day.  For  this  reason  the  verti- 
cal interval  between  the  terraces  and  the  present  streams 
which  they  follow — that  is,  the  height  of  the  downward- 
sloping  escarpments  bordering  them  on  their  valley  margins — 
progressively  decreases  from  the  gateways  in  the  mountain 
to  the  sea.  The  lower  terraces  composed  of  soft  material 
far  out  on  the  plains  have  melted  down  and  been  removed 
to  a  great  extent,  under  the  action  of  the  atmosphere,  while 
the  higher  terraces  in  firmer  material  have  retained  their 
characteristic  topographic  forms. 

In  the  terraces  of  the  Rocky  Mountains  and  adjacent 
portions  of  the  Great  Plains,  the  wide-reaching  influence  of 
the  Glacial  epoch  is  again  recorded.  The  Rocky  Mountains 
had  their  local  glaciers  at  that  time,  and  the  streams  were 


'!i 


''"(■PP 


« 


% 


i?-%!ii- 


178 


RIVERS  OF  NORTH  AMERICA 


flooded  especially  during  the  final  melting  of  the  ice,  when, 
it  is  inferred,  the  rain-fall  was  also  abundant,  but  the  swol- 
len streams  were  overloaded  and  their  channels  became 
deeply  filled.  The  effects  of  more  or  less  periodic  changes 
in  the  elevation  of  broad  areas  above  the  sea  may  perhaps 
be  made  out  in  this  region  from  the  terrace  records,  but  as 
yet  too  little  attention  has  been  given  to  this  subject,  and 
in  fact  to  the  general  surface  features  of  the  Rocky  Mountain 
region,  to  warrant  one  in  offering  anything  rhore  than  pro- 
visional answers  to  the  questions  here  suggested. 

In  the  Great  Basin  region  variations  of  climate  are  again 
recorded  by  the  work  of  the  s'  reams.  Changes  in  the  rate 
at  which  streams  deposit,  it  will  be  remembered,  depend  on 
changes  in  velocity  and  in  load.  Velocity  depends  in  part 
on  volume.  A  change  in  climatic  conditions  from  arid  to 
humid  means  an  increase  in  the  volume  of  the  draining 
streams.  Whether  this  increment  in  velocity  will  be  ac- 
companied by  deeper  corrasion  or  by  aggrading,  is  deter- 
mined by  the  accompanying  change  in  the  rate  at  which 
the  streams  are  loaded.  An  increased  rain-fall  would  be 
accompanied  by  a  greater  removal  of  loose  material  from 
the  highlands,  and  consequently  a  greater  contribution  of 
debris  to  the  stream.  There  would,  therefore,  be  a  bal- 
ancing of  conditions  in  any  one  part  of  a  stream's  course. 
If  the  supply  of  debris  was  not  in  excess  of  the  transporting 
power  of  the  draining  streams,  they  would  corrade,  but  if 
the  loads  delivered  to  them  were  too  great,  deposition 
would  result.  When  the  whole  extent  of  a  river  is  con- 
sidered, a  change  from  arid  to  humid  conditions  must 
increase  both  corrasion  and  deposition.     More  active  cor- 


::^S|i!iii!il 


STREAM   TERRACES 


179 


rasion  in  the  upper  portion  of  a  drainage  system  usually 
necessitates  a  greater  rate  of  deposition  lower  down. 

The  streams  of  the  Great  Basin  felt  the  effects  of  the 
change  of  climate  which  caused,  or  accompanied,  the  Gla- 
cial epoch,  and,  as  the  results  indicate,  were  overloaded. 
Many  of  them  are  bordered  by  terraces  in  such  a  manner  as 
to  show  that  they  were  formerly  of  greater  volume  than  at 
present,  and  subsequently  decreased  in  volume,  but  were 
able  for  a  time  to  cut  channels  and  broaden  their  valleys  in 
previously  formed  flood-plains.  In  many  instances  the 
streams  have  diminished  to  such  an  extent,  however,  that 
they  are  now  aggrading.  In  their  enfeebled  condition  they 
are  unable  to  carry  even  the  small  burdens  that  are  imposed 
upon  them.  _. 

In  the  region  of  the  high  plateaus  drained  by  the  Colo- 
rado and  its  branches,  there  are  many  terraces.  The 
majority  of  those  which  attract  the  eye,  however,  are  due 
to  the  weathering  of  the  outcrop  of  alternating  hard  and 
soft  strata,  but  stream  terraces  also  occur.  Many  broad 
valleys  are  deeply  filled  and  without  terraces,  as  is  illustrated 
by  Fig.  A,  Plate  XVI.,  owing  to  the  fact  that  aggrading  has 
been  in  progress  throughout  a  large  portion  of  the  present 
arid  period. 

The  Colorado  River,  as  is  well  known,  flows  through  a 
canyon  within  a  canyon.  The  outer  canyon  is  in  places 
some  fifteen  miles  broad  and  comparatively  level-floored. 
Sunken  in  this  floor  is  the  deeper,  inner  canyon,  which  in 
places  is  from  one  to  two  miles  broad.  The  floor  of  the  outer 
canyon  is  thus  a  great  terrace,  as  may  be  seen  on  inspect- 
ing Plate  XVII.     The  general  history  to  be  read  in  these 


^Pl 


1 80 


RIVERS  OF  NORTH  AMERICA 


features  is  that  the  region  was  at  one  time  lower  than  now 
by  an  amount  about  equal  to  the  depth  of  the  inner  canyon. 
The  Colorado  cut  down  its  channel  to  baselevel  and  then 
broadened  it  into  a  wide  valley.  Subsequent  elevation 
renewed  the  energy  of  the  stream  and  enabled  it  to  cut 
down  nearly  to  baselevel  once  more,  leaving  large  portions 
of  the  bottom  of  its  older  valley  as  a  terrace  on  each  side  of 
its  later  gorge.  The  terrace  thus  formed  extends  into 
many  of  the  tributaries  of  the  main  river,  and  in  fact  the 
change  to  which  it  was  due  affected  a  large  part  if  not  the 
entire  drainage  system.  \-,'  -s;.--^':-:,.^;^::^':,: 

Our  knowledge  of  the  terraces  in  the  valleys  of  North 
America  is  too  immature  to  permit  us  to  state  with  confi- 
dence their  distribution  throughout  the  continent,  and  one 
more  illustration  must  sufifice  for  this  brief  review. 

Columbia  River,  in  the  portion  of  its  course  known  as 
the  Big  Bend,  flows  through  a  narrow,  canyon-like  valley 
on  the  side  of  which  there  are  numerous  terraces.  A  short 
account  of  these  taken  from  a  report  by  the  present  writer,' 
will  indicate  not  only  the  nature  of  the  stream  records  so 
abundant  on  the  sides  of  many  of  the  canyons  and  valleys 
of  the  far  Northwest,  but  also  show  how  stream  terraces 
are  frequently  associated  with  similar  topographic  forms 
originating  in  other  ways: 


"  On  descending  the  side  of  the  canyon  [of  the  Columbia,  op- 
posite the  entrance  of  Chelan  valley]  by  means  of  a  road  following 
a  deep,  high-grade  gorge,  we  notice  that  there  are  many  terraces 
on  each  side  of  the  river.     The  most  remarkable  of  these,  and 

'  I.  C.  Russell,  "A  Geological  Reconnoissance  in  Central  Washington,"  U. 
S.  Geological  Survey,  Bulletin  No.  108,  pp.  78,  79,  1893. 


^m:. 


STREAM    TERRACES 


i8i 


one  of  the  finest  examples  of  terrace  structure  that  can  be  found 
anywhere,  is  a  level-topped  shelf  formed  of  gravel  and  water- 
worn  boulders,  the  surface  of  which  is  seven  hundred  feet  above 
the  Columbia.  This  truly  remarkable  terrace  is  best  developed 
about  two  miles  below  where  we  descended  into  the  canyon.  It 
is  t'  ere  several  hundred  feet  broad  and  runs  back  into  lateral 
gorges,  showing  that  the  sides  of  the  main  canyon  were  deeply 
scored  by  lateral  drainage  before  the  gravel  forming  the  terrace 
was  deposited.  On  the  west  side  of  the  valley  there  are  other 
fragments  of  the  same  deposits,  forming  a  less  conspicuous  shelf, 
which  has  been  built  against  the  steep  slope,  and  has  the  same 
level  as  the  great  terrace  on  the  east  side  of  the  river.  The 
valley  was  excavated  lower  than  the  bottom  over  which  the 
Columbia  now  flows,  and  then  filled  in  from  side  to  side  with 
stream-borne  stones,  gravel,  and  sand  before  the  present  channel 
was  excavated.  In  the  re-excavation,  fragments  of  the  deposits 
filling  the  canyon  have  been  left  clinging  to  its  slopes.  Streams 
flowing  down  lateral  go  ges  have  cut  channels  across  the  terrace, 
thus  revealing  the  structure  even  more  plainly  than  the  steep 
slope  leading  to  the  river. 

"  Above  the  valley  opening  in  the  west  wall  of  the  canyon 
and  leading  to  Lake  Chelan,  there  are  other  large  remnants 
of  the  same  great  terrace,  this  time  on  the  west  side  of  the 
river.  In  the  broad  plain  formed  by  the  surface  of  the  terrace, 
there  stands  a  lofty  pyramid  of  solid  rock  completely  surrounded 
by  the  gravel  deposit  and  rising  like  an  island  from  its  level 
surface. 

"  The  terrace  gravels  extend  into  the  valley  of  Lake  Chelan 
and  form  conspicuous  terraces  about  its  lower  end.  For  many 
miles  both  up  and  down  the  Columbia,  other  fragments  of  the 
same  level-topped  deposit  occur,  always  forming  striking  features 
in  the  landscape,  owing  to  the  marked  contrast  of  their  smooth 
horizontal  lines  with  the  vertical  line  due  to  the  erosion  of  rills 
and  creeks. 

"  Beside  the  great  terrace  described  above  there  are  many 
other  but  less  conspicuous  horizontal  lines  on  the  sides  of  the 
Columbia  canyon.     Some  of  these  below  the  horizon  of  the  main 


!1 

if" 


l82 


RIVERS  OF  NORTH  AMERICA 


II, 


•<* 


sr; 


terrace  are  stream  terraces,  made  by  the  river  in  lowering  its  bed. 
A  more  numerous  but  much  less  regular  class  are  due  to  land- 
slides, of  wh  ch  there  have  been  many.  Other  horizontal  lines 
are  due  to  the  unequal  weathering  of  the  strata  of  basalt  and  of 
interstratified  sedimentary  beds. 

"  Still  another  class  of  terraces,  both  numerous  and  conspicu- 
ous, has  been  formed  as  moraines  on  the  sides  of  the  glacier 
that  once  filled  the  canyon  up  to  an  elevation  of  1200  feet  above 
the  river  as  it  flows  to-day.  The  moraine  terraces  are  of  older 
date  than  the  great  terrace  described  above,  and  about  the  en- 
trance of  Chelan  valley  have  been  partially  buried  by  it. 

*'  In  the  canyon  of  the  Columbia  for  several  miles  above  Lake 
Chelan  its  rugged  sides  zct  strewn  with  thousands  of  perched 
boulders,  left  by  the  retreat  of  the  ice.  These  have  a  definite 
upper  limit,  but  mingled  with  them  are  masses  of  basalt  thai  have 
fallen  quite  recently  from  the  cliffs  ab^ve. 

"  In  embayments  along  the  sides  of  the  main  canyon  and  back 
of  the  ridges  of  stone  and  boulders  left  by  the  ancient  glaciers, 
there  are  flat  areas  which  have  been  filled  in  with  fine  material, 
washed  from  higher  levels.  These  plains  have  in  some  instances 
been  cut  by  small  streams  flowing  across  them,  thus  adding  other 
horizontal  lines  to  the  complex  topography  of  the  canyon 
walls. 

"  It  is  not  practicable  to  describe  these  terraces  in  detail,  but 
those  who  visit  Lake  Chelan  will  have  an  opportunity  to  read  for 
themselves  the  re?.iarkable  history  which  they  record.  In  study- 
ing them,  however,  the  traveller  must  bear  in  mind  that  the 
canyop.  ""'ter  bcig  cut  through  various  rocks  to  a  depth  greater 
tiian  it  now  h£v  was  occupied  by  a  large  gUcier,  and  then  by  an 
arm  of  a  large  lake,  and  that  river,  glacial,  and  lacustral  records 
are  inscribed  on  the  same  slope.  In  addition,  there  have  been 
many  landslides,  producing  deceptive,  terrace-like  forms,  and 
terraces  due  to  the  unequal  weathering  of  hard  and  soft 
beds." 

A  continuation  of  this  review  of  the  distribution  of  ter- 
races   would    lead    us   northward   to   the    Mackenzie   and 


STREAM   TERRACES 


183 


d 


)f  ter- 
and 


Yukon,'  where  many  records  similar  to  those  just  con- 
sidered are  known  to  exist,  but  our  present  knowledge  of 
the  changes  these  streams  have  experienced  is  even  more 
scanty  than  for  the  central  and  southern  portions  of  the 
continent. 

The  most  important  result  of  this  hasty  excursion  through 
terraced  river-valleys  is  perhaps  the  recognition  of  the  fact 
that  terraces  exist  along  the  sides  of  stream-cut  valleys 
throughout  the  lengtii  and  breadth  of  North  America. 
Volumes  of  history  are  recorded  in  those  graceful  curves 
which  give  beauty  to  the  varied  scenery  of  valley  borders 
from  the  tropical  forests  of  Central  America  and  Mexico  to 
beyond  the  Arctic  circle.  The  interpretation  of  these 
records  has  only  recently  been  undertaken,  and  much  that 
is  new  unquestionably  awaits  the  patient  explorer.  The 
general  principles  to  be  used  in  this  study  have  been  pre- 
sented in  the  present  chapter,  but  as  investigation  pro- 
gresses, much  that  is  novel  in  details,  and  orobably  also  the 
discovery  of  as  yet  unknown  laws  or  modifications  of  those 
now  recognised,  will  reward  the  student. 

'  I.  C.  Russell,  "  Notes  on  the  Surface  Geology  of  Alaska,"  in  Bulletin  of 
the  Geological  Society  of  America,  vol.  i.,  pp.  144-146,  1890. 


:»^l 


I* 


CHAPTER  VII 
STREAM  DEVELOPMENT 

Consequent  Streams. — In  case  a  portion  of  the  sea  floor 
should  be  upraised  so  as  to  make  what  geographers  term  a 
new  land-area,  the  streams  flowing  from  it  would  take  the 
easiest  courses,  as  detennmed  by  the  slope  of  the  surface, 
regardless  of  the  structure  of  the  rocks  beneath.  If  a 
dome-shaped  uplift  should  occur  in  a  broad  plain  underlain 
by  previously  horizontal  layers  of  rock,  the  rain  falling  on 
its  surface  would  form  rills,  and  these  uniting  would  give 
origin  to  creeks  and  perhaps  to  rivers,  which  would  flow 
away  in  all  directions  from  the  sunimit  portion  of  the  uplift. 
In  each  of  these  hypothetical  cases  the  streams  would  evi- 
dently have  their  directions  determined  by  the  pre-existing 
topography,  and  hence  may  be  termed  conseipient  streams. 

Subsequent  Streams. — As  consequent  streams  deepened 
their  channels  they  might  discover  differences  in  the  rate 
at  which  they  can  remove  the  rocks,  and  hence  have  their 
directions  variously  modified  by  differences  in  hardness  or 
by  the  greater  or  less  solubility  of  the  various  beds  that 
they  encountered.  New  branches  or  tribu':aries  to  the  main 
streams  would  be  developed,  the  positions  of  which  would 
be  determined  by  the  down-cutting  of  the  channels  of  the 

184 


STREAM  DEVELOPMENT 


185 


master  streams,  and  by  the  relative  ease  with  which  the 
various  rocks  coming  to  the  surface  could  be  removed. 
That  is,  as  a  drainage  system  develops,  streams  originate, 
the  directions  of  which  are  r  I'ated  by  the  hardness  and 
solubility  of  the  rocks.  Such  streams  appear  subsequently 
to  the  main  topographic  features  in  their  environment,  and 
are  iGrmtd  subsequent  streams. 

Ideal  lilustration  of  Stream  Adjustment  and  Development. 
— Perhaps  the  best  way  in  which  to  obtain  a  graphic  idea 
of  the  changes  passed  through  by  a  river  system  in  the 
course  of  its  development  and  adjustment  to  the  geological 
conditions  it  discovers,  when  unaffected  by  marked  climatic 
changes  and  not  seriously  disturbed  by  movements  in  the 
earth's  crust,  is  to  form  a  mental  picture  of  a  gently  sloping 
plateau  with  an  essentially  even  surface  on  which  rain  falls 
and  gives  origin  to  rills  and  brooks  which  unite  to  form 
larger  consequent  streams,  and  picture  to  ourselves  the 
changes  that  would  result  under  the  influence  of  a  moder- 
ately humid  climate.  Imagine  such  a  plateau,  we  will  say, 
one  hundred  miles  long  from  right  to  left  and  fifty  miles 
broad,  sloping  gently  toward  an  assumed  point  of  view,  and 
follow  in  fancy  the  changes  in  the  streams,  and  the  result- 
ing modifications  in  the  relief  of  the  surface  as  normal 
stream  development  progresses.  To  complete  our  concep- 
tion, we  may  assume  that,  beyond  the  sky-line  bounding  the 
far  side  of  the  landscape,  the  surface  gently  declines  in 
the  opposite  direction.  That  is^  the  plateau  before  us  is 
the  side  of  a  long  ridge,  the  central  axis  of  which  is  raised, 
we  will  say,  one  thousand  feet  above  sea-level. 

The  rocks  beneath  the  surface  of  the  lilted  plane,  we  will 


K\ 


I15 


if. 


I 


i,Mlfl 


i!i|iiiii 


1 86 


RIVERS  OF  NORTH  AMERICA 


assume,  are  in  inclined  layers,  which  slope  toward  our  point 
of  view,  at  a  greater  angle  than  the  surface  of  the  plane,  and 
besides  are  composed  of  hard  and  soft  beds.  A  section 
through  the  tilted  plane  at  right  angles  to  the  axis  of  the 
uplift  has  the  structure  indicated  in  the  following  diagram. 


Fig.  15.     Section  at  Right  Atn^les  to  the  Lines  A  A  and  B  B  in  Fig.  16. 

The  harder  rocks  are  shaded.  The  lines  in  which  the  inclined 
beds  join  the  surface  of  the  plane  run  in  the  direction  of  its 
length.  The  conditions  here  assumed  aivi  tn^  as  might 
result  if  a  region  underlain  by  inclined  stratihed  rocks  had 
been  planed  away  nearly  to  baselevel  and  then  upraised  so 
as  to  produce  a  gently  sloping  peneplain. 

The  rain-water,  falling  on  the  surface  of  the  plane  before 
us,  gathers  in  part  in  depressions  on  its  surface  and  fo'*ms 
ponds  and  lakes.  These  are  but  temporary,  however,  .is 
the  shallow  basins  are  soon  filled  with  sediment,  or  have 
their  outlets  tut  down  by  the  overflowing  water,  and  are 
drained.  The  rills  supplied  directly  from  the  rain  and  those 
starting  from  the  lakelets  unite  to  form  larger  streams,  *,»!( :h 
flow  down  the  inclined  plane  to  the  sea  \\\  obedf':**  c 
gravity.  The  directions  of  these  initial  streams  are  de.., 
mined  by  the  slope  of  the  surface.  Thc}^  are,  therefore, 
consequent  streams.  Their  number  will  be  determined  by 
the  inequalities  of  the  surface  which  cause  the  rills  and 
rivulets  to  unite.  Some  will  be  longer  than  others.  Some 
will  have  a  greater  volume  than  others.  A  common  feature, 
shared  at  first  by  all,  is  that  they  have  but  few  tributaries, 


STREAM  DEVELOPMENT 


l87 


A  diagram   showing   these  initial  consequent  streams,   in 
their  infancy,  is  presented  in  the  following  ideal  map.' 


Fig.  i6.     Ideal  Sketch-Map  Showing  Young  Consequent  Streams. 

The  consequent  streams,  a  to  i,  follow  courses  determined 
by  the  slope  of  the  surface  and  approximately  straight.  The 
h?-dness  or  softness  of  the  underlying  rock  does  not  affect 
them  at  first,  for  the  reason  that  they  are  surface  streams. 
They  differ  in  volume,  as  is  indicated  to  some  extent  by  the 
length  of  the  lines  representing  them  in  the  diagram.     All 

'  Figures  i6,  17,  and  18,  together  with  almost  all  of  the  account  of  the 
development  of  streams  here  presented,  have  been  taken  from  a  highly  instruc* 
tive  article  by  W.  M.  Davis,  on  "  The  Development  of  Certain  English  Rivers," 
ir  Tk(  Geo^'-aphical  Journal  {o{  the  Royal  Cieographical  Society),  vol.  v.,  pp. 
127-1^6.     Ljndon,  1895. 


Ui 


ir 


i  • 

14 

'If 


1 88 


RIVERS  OF  NORTH  AMERICA 


of  them  begin  at  once  the  task  of  deepening  their  channels 
to  a  certain  grade  determined  by  their  volumes  and  their 
loads.  This  work  would  not  progress  at  the  same  rate  in 
all  the  streams  because  they  are  of  unequal  length,  of  differ- 
ent volumes,  and  are  variously  loaded.  The  longer  streams 
would,  under  most  conditions,  be  the  larger,  and  would 
corrade  their  channels  most  rapidly.  The  drainage  from 
the  inter-stream  spaces  flows  to  the  master  streams  and  de- 
velops feeding  branches.  The  gradients  of  these  branches 
depend  on  the  rate  at  which  the  master  streams  deepen  their 
channels.  The  branches  of  the  more  rapidly  corrading 
master  streams  have  their  velocities,  and  consequently 
theif  corrading  power,  increased  more  rapidly  than  the 
similar  branches  of  their  weaker  neighbours.  Hence  the 
branches  of  the  stronger  consequent  streams  cut  back  by 
head-water  corrasion  and  increase  in  length  more  rapidly 
than  the  branches  of  the  weaker  consequent  streams.  More 
and  more  of  the  water  falling  on  the  territory  between  the 
main  streams  is  thus  carried  to  the  more  favoured  conse- 
quent streams,  and  increases  still  more  their  advantage 
over  their  weaker  neighbours. 

The  initial  streams,  it  will  be  remembered,  flowed  down 
the  original  slope,  as  shown  in  Fig.  15,  at  right  angles  to 
the  strike,  that  is,  across  the  edges  of  the  strata  composing 
the  tilted  block  of  the  earth  crust  we  have  in  mind ;  the 
branches  of  the  streams,  however,  flow  parallel  with  the  edges 
of  the  strata  where  they  come  to  the  surface,  and  hence 
find  hard  and  soft  bands  parallel  with  their  courses.  As 
erosion  progresses,  the  edges  of  the  resistant  beds  are  left 
in  relief,  forming  ridges,  while  the  less  resistant  beds  arc 


STREAM  DEVELOPMENT 


189 


more  rapidly  removed  ana  determine  the  courses  of  the 
subsequent  branches.  As  this  process  goes  on,  our  sloping 
plane  loses  its  smoothness,  channels  are  cut  by  the  conse- 
quent streams,  and  depressions  are  made  by  their  branches 
trending  in  general  at  right  angles  to  their  courses.  Be- 
tween  the  lateral  valleys  ridges  appear,  marking  the  posi- 


KlG.  17.     Ideal  Sketch-Map  Illustrating  Stream  Development. 

tions  of  the  edges  of  the  resistant  beds.  These  ridges  are 
at  first  straight,  and  mark  the  intersection  of  the  hard  beds 
with  the  original,  gently  sloping  surface.  As  the  strata  are 
inclined,  however,  one  side  of  a  ridge  fcmed  by  the  out-, 
cropping  edge  of  a  hard  bed  will  have  a  more  gentle  slope 
than  the  opposite  side.     The  sides  oi  the  ridges  slope  gently 


I 


!lf:i 


190 


RIVEHS  OF  NORTH  AMEPICA 


toward  our  assumed  point  of  view  and  present  steep  escarp- 
ments in  the  opposite  direction.  The  condition  of  the 
sloping  surface  before  us  at  this  stage  is  shown  in  Fig.  17. 
The  hard  layers,  indicated  by  broken  bands,  stand  up  as 
ridges,  and  the  branches  of  the  original  consequent  stream 
have  begun  to  levelop  valleys  in  the  soft  rocks. 


Fig.  18.     Ideal  Skelch-Map  Showing  an  Advanced  Stage  in  Stream  Develop- 
ment.     Former  Shore  Shown  by  Broken  Line 

The  consequent  stream  c,  being  stronger  and  corradiny^ 
more  rapidly  than  a,  deepens  its  channel  through  the  hard 
ridge  A  A  more  rapidly  than  its  weaker  neighbour.  The 
branch  m'  of  the  strong  consequent  stream  c,  having  its 
place  of  discharge  lowered  by  the  corrasion  of  the  stream 


)  escarp- 
of  the 
Fig.  17. 
id  up  as 
t  stream 


r 


y 


m 


III 


,  X  M   I  .  .   V^, 


N. 


J 


i\  Develop- 

lorradini; 

:he  hard 

ir.     The 

[ving  its 

stream 


S  TREA  M  DE  VE  LOP  MEN  T 


191 


to  which  it  is  tributary,  is  able  to  remove  the  soft  rocks 
forming  its  bed  and  to  grow  in  length  by  head -water  corra- 
sion  more  rapidly  than  the  corresponding  subsequent  branch 
of  a.  As  m'  increases  in  length,  it  captures  more  and  more 
of  the  drainage  of  a,  thus  weakening  that  stream,  and  at 
length  draws  off  all  of  its  water  above  o.  The  original 
stream  a  is  thus  broken  in  two,  or  is  beheaded,  to  use  a 
term  proposed  by  Davis.  The  notch  that  a  has  cut  in  the 
ridge  of  hard  rock  A  A  is  left  as  a  wind-gap.  The  bottom 
of  this  gap  is  a  divide  from  which  the  waters  flow  each  way. 
The  beheaded  stream  a'  holds  its  former  course  below  the 
divide,  but  is  weakened  by  the  loss  of  its  head-waters; 
the  portion  of  the  stream  a,  from  the  divide  on  the  hard 
bed  to  where  the  subsequent  stream  m'  intersected  it,  is 
reversed.  For  such  reversed  streams  Davis  has  proposed  the 
name  obseqnent.  As  time  goes  on,  changes  similar  to  those 
accompanying  the  backward  cutting  of  in'  take  place  also  in 
the  other  streams,  as  may  be  seen  from  Fig.  18,  in  which 
a  more  advanced  stage  in  stream  development  is  indicated. 
Another  feature  of  the  changes  in  progress  is  shown  by 
the  fact  that  the  ridges  of  hard  rock,  indicated  by  the 
bands  A  A  and  B  B,  are  no  longer  straight.  When  the 
stronger  streams  cut  through  them,  forming  water-gaps, 
the  cliffs  recede  by  weathering  and  by  the  sapping  of  their 
bases  by  the  streams.  This  wasting  of  the  hard  ridges  goes 
on  most  rapidly  on  the  side  toward  which  they  slope  most 
steeply;  that  is,  on  the  farther  side  from  our  assumed  point 
of  view.  The  cliffs,  formed  by  the  steeper  slope  of  the 
ridges  of  hard  rock,  thus  gradually  migrate  in  the  direction 
of  the  dip  of  the  hard  beds,  that  is,  toward  our  point  of 


^m^ 


|.;;:' 


192 


RIVERS  OF  NORTH  AMERICA 


\m 


view,  under  the  influence  of  general  erosion  and  sapping 
throughout  their  entire  length.  But  this  recession  is  most 
rapid  where  the  stronger  consequent  streams  cross  them, 
and  they  become  lobed  or  scalloped,  as  shown  in  Fig.  17. 
With  the  process  of  stream  development,  the  alignment  of 
the  cliffs  becomes  more  and  more  modified,  until  the  reces- 
sion in  the  neighbourhood  of  the  master  stream  is  checked 
by  the  streams  having  cut  down  nearly  to  baselevel.  At 
this  stage,  the  cliffs  at  the  ends  of  the  V-shaped  gorges,  at 
the  apex  of  which  the  master  streams  cross  the  hard  beds, 
will  remain  stationary  and  crumble  away,  while  those  por- 
tions of  the  cliffs  between  the  master  streams,  which  before 
receded  more  slowly  than  the  portions  near  the  stream,  will 
retreat  more  rapidly  than  the  portions  of  the  escarpment 
near  where  the  master  streams  have  cut  to  baselevel.  In 
an  advanced  stage  of  stream  adjustment  and  of  topographic 
development,  the  lines  of  cliffs,  at  first  straight  and  then 
deeply  '  )bed,  will  again  approach  an  even  alignment,  but 
the  position  of  the  ridges  will  change  with  this  development, 
and  move  in  the  direction  of  the  dip  of  the  hard  beds.  The 
cliffs  in  an  early  stage  of  development  were  of  faint  relief; 
when  cut  into  deep  lobes,  they  stand  up  prominently,  but 
as  their  alignment  is  again  established,  they  become  sub- 
dued, and  when  the  process  is  far  advanced  nearly  or  quite 
disappear. 

To  return  to  the  development  of  drainage.  The  manner 
in  which  the  subsequent  stream  m! ,  Fig.  17,  captured  and 
diverted  the  head-water  of  a  and  divided  that  stream  into  a 
beheaded  portion,  a\  a  reversed  portion,  0,  and  a  diverted 
portion,  a" ,  will  serve  to  illustrate  the  similar  process  fol- 


STREAM  DEVELOPMENT 


193 


I 


lowed  by  other  streams.  In  each  instance,  the  subsequent 
branches  of  the  stronger  primary  stream  cut  back  until  they 
divert  a  portion  of  the  drainage  of  their  weaker  neighbours. 
The  result  of  this  process  can  be  easily  predicted  from  a 
study  of  the  map.  At  a  certain  stage  in  the  process,  the 
stronger  streams  c  and  //,  will  have  captured  the  head-water 
of  ail  of  their  rivals  above  the  ridge  A  A,  and  a  competition 
between  the  two  conquering  streams  will  ensue.  An  ad- 
vanced stage  in  this  struggle  is  indicated  in  Fig.  18,  where 
the  subsequent  branches  ;;/"  and  n"  of  the  stronger  mas- 
ter stream  c  have  captured  and  diverted  the  head-waters 
of  //. 

In  the  map  forming  Fig.  18,  it  will  be  noted  that  the 
ridges  of  hard  rock  are  again  nearly  straight,  and  also  that 
the  lower  courses  of  c  and  h  have  become  tortuous.  The 
reason  for  the  curves  in  the  lower  courses  of  the  stronger 
stream  is  that  after  cutting  down  their  channels  nearly  to 
baselevel  and  becoming  sluggish,  they  continue  to  corrade 
laterally  and  form  flood-plains  on  which  they  meander  from 
side  to  side,  at  the  same  time  broadening  their  valleys. 
As  has  been  shown  on  a  previous  page,  a  sluggish  stream, 
having  little  power  to  overcome  obstacles,  is  more  easily 
deflected  than  a  swifter  stream,  and  besides,  the  lower  por- 
tions of  the  valley  and  plains  tracts  of  a  stream  during  ad- 
vanced stages  in  development  are  regions  of  deposition. 
The  migrations  of  the  streams  a  and  c  have  brought  them 
together,  as  shown  in  Fig.  18,  thus  illustrating  another 
process  of  capture. 

The  examples  of  stream  development  we  have  been  fol- 
lowing are  ideal,  but,  I  believe,  true  to  nature.     The  reason 


!H 


»3 


194 


RIVERS  OF  NORTH  AMERICA 


for  sketching  an  ideal  illustration  is  that  in  nature  various 
disturbing  conditions  usually  modify  the  process  and  in- 
crease the  difficulty  of  separating  what  is  normal  from  that 
which  may  be  termed  accidental.  Stream  development  is 
a  slow  process  even  when  not  disturbed  by  marked  climatic 
changes  or  by  movements  in  the  earth's  crust.  The  life  of 
a  man,  or  even  of  a  nation,  is  too  short  to  embrace  the  time 
necessary  for  the  development  of  a  river  system.  It  is  only 
by  studying  many  streams  in  various  stages  of  their  develop- 
ment that  geographers  are  able  to  sketch  generalised  pictures 
of  the  normal  changes  a  great  river  passes  through  in  its  life- 
span of  millions  of  years. 

V\^hen  one  attempts  to  apply  the  elementary  conceptions 
of  stream  development  outlined  above  to  actual  streams,  it 
is  found  that  many  modifying  conditions  have  to  be  taken 
into  account.  Broad  surfaces  with  even  initial  slopes  are 
rare;  the  rocks  forming  the  earth's  crust  are  frequently 
folded  and  faulted,  especially  in  uplifted  regions,  and  con- 
sequently the  development  of  subsequent  streams  is  fre- 
quently greatly  modified ;  more  puzzling  complications 
arise,  however,  from  the  fact  that  the  land  is  not  stable, 
but  is  subject  to  up-and-down  movements,  which  disturb  or 
entirely  arrest  the  slow  process  of  stream  c''=;velopment  be- 
fore it  can  run  its  normal  course.  These  and  still  other 
modifying  conditions  have  to  be  considered  in  studying  the 
histo"'  of  the  streams  which  drain  the  land  and  throughout 
their  history  are  continually  modifying  the  relief  of  the 
•surface. 

With  the  introduction  to  the  principle  of  stream  develop- 
ment in  mind,  let  us  turn  to  a  portion  of  our  own  land 


S  TREA  M  DE  VEL  OP  MEN  T 


«95 


where  the  processes  just  outlined  have  been  long  in  action 
and  see  if  we  can  read  a  portion  of  the  history  recorded  by 
the  valley  and  intervening  hills.         '  '  ; 

EXAMPLES  OF  STREAM    DEVELOPMENT  AND   ADJUSTMENT 
IN   THE   APPALACHIAN    MOUNTALNS 

The  leading  geographical  feature  of  the  Appalachian 
Mountains,  more  especially  of  their  northern  half,  is  the 
large  number  of  curving  but  generally  parallel,  level-topped 
ridges  with  valleys  between,  which  compose  the  uplifted 
region.  Crossing  the  ridges  and  valleys  approximately  at 
right  angles  is  a  series  of  rivers,  such  as  the  Delaware,  Sus- 
quehanna, Potomac,  and  James,  which  have  their  sources  to 
the  west  of  the  mountains,  and  flow  eastward  to  the  sea. 
These  master  streams  receive  many  branches  from  the  valleys 
crossed  by  them.  The  general  features  referred  to  are 
shown  on  the  map  forming  Plate  IX. 

The  ridges  in  the  northern  Appalachians  are  known  to 
be  due.  to  folds  in  the  rocks,  which  have  been  truncated  or 
planed  off  to  a  certain  general  level,  and  the  surface  thus 
formed  upraised  and  eroded  so  as  to  leave  the  edges  of  hard 
beds  in  bold  relief.  The  first  question  suggested  by  an  in- 
spection of  the  accompanying  map  is :  How  has  it  come  about 
that  the  main  rivers  flow  through  the  ridges  of  hard  rock  by 
means  of  gaps  cut  in  them,  instead  of  being  turned  aside 
and  pursuing  what  would  seem  to  be  much  easier  courses  to 
the  sea?  The  ideal  case  of  river  development  we  now  have 
in  mind  will  assist  in  solving  this  problem.       

The  study  of  the  northern  Appalachians,  conducted  by 
a  large  number  of  geologists,  and  especially  by  Davis  and 


1  iviW^'lM 


196 


mVEIiS  OF  NORTH  AMERICA 


Willis,  has  shown  that  after  the  rocks  were  folded  the  region 
existed  as  a  land  area,  probably  more  elevated  than  now, 
and  was  worn  down  nearly  to  sea-level,  or,  in  other  words, 
was  reduced  to  the  condition  of  a  peneplain.  This  pene- 
plain was  subsequently  elevated  and  tilted  so  as  to  slope 
toward  the  south-east.  The  conditions  were  then  essentially 
the  same  as  in  the  ideal  case  already  discussed,  except  that 
the  rocks  beneath  the  tilted  plane  had  a  complex  structure. 
The  rocks  were  also  of  many  degrees  of  hardness  and  solu- 
bility. The  south-eastern  margin  of  this  tilted  peneplain 
was  at  sea-level,  while  its  north-western  border,  in  the  region 
now  embraced  in  the  central  and  western  portion  of  New 
York,  Western  Pennsylvania,  and  West  Virginia,  was  ele- 
vated, not  all  at  once,  but  slowly,  to  a  hei"-ht  of  probably 
two  or  three  thousand  feet. 

We  have  designated  the  sloping  surface  referred  to  as  a 
tilted  plane,  more  accurately  it  should  be  considered  as  the 
side  of  an  elongated  dome-Uke  uplift. 

The  pre-existing  streams  flowing  with  slack  currents  in 
their  old  age,  and  young  consequent  streams  originating  on 
the  tilted  peneplain,  took  the  direction  of  easiest  descent, 
and  flowed  south-eastward  to  the  sea.  The  courses  of  these 
streams  were  determined  by  the  slope  of  the  surface  irre- 
spective of  the  position  or  character  of  the  rocks  beneath, 
and  hence  are  consequent  streams.  As  they  deepened  their 
channels,  the  edges  of  hard  and  soft  beds  were  cut  through. 
With  this  process  of  deepening  the  channels  of  the  conse- 
quent streams,  many  subsequent  branches  originated  which 
also  entrenched  themselves,  but  their  directions  were  con- 
trolled by  the  hardness  or  solubility  of  the  rocks.     Those 


he  region 
lan  now, 
er  words, 
his  pene- 

to  slope 
isentially 
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tructure. 
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le  region 

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d  to  as  a 
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rents  in 
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,\|)|)rii\iiii.it'  vil'   '^•"iv-^ou  II 


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oilhein    \l'rii>'B"<T  llailey  Willis.) 
i|iriixininli  iCiilf   ■^•"iv-wmh  nil|p». 


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STREAM  DEVELOPMENT 


197 


originating  on  soft  beds  maintained  their  positions,  while 
those  which  flov^ed  at  first  along  the  outcrops  of  hard  beds 
were  soon  shifted  to  the  softer  beds  adjacent.  The  hard 
beds  were  thus  left  as  ridges  between  the  valleys  excavated 
by  the  subsequent  streams  along  the  outcrops  of  soft  beds. 
As  the  master  streams  flowing  south-eastward  lowered  their 
channels  their  subsequent  branches  were  given  greater 
velocity  and  deepened  their  channels  also.  This  sinking 
of  the  rivers  and  of  all  their  branches  produced  a  roughening 
of  the  topography  and  the  once  nearly  smooth  plain  became 
a  rugged  mountainous  region.  In  this  general  down-cutting 
the  large  consequent  streams  first  reached  baselevel  at  their 
mouths,  and  then  a  low  gradient,  or  an  approximation  to 
baselevel,  was  produced  progressively  up  stream.  The  tend- 
ency of  all  the  streams,  or  their  chief  aim,  as  we  may  say, 
was  to  reduce  the  land  to  a  second  baselevel.  This  would  be 
accomplished  by  the  downward  corrasion  of  the  streams  in 
their  upper  courses,  and  a  broadening  of  their  valleys  in 
their  lower  courses ;  the  broadening  process  progressmg  up 
stream  as  fast  as  the  valleys  were  deepened  at  a  certain 
progressively  decreasing  rate. 

During  the  process  outlmed  above,  each  <>f  the  subsequent 
branches  of  the  main  streams  entered  into  competition  with 
its  neighbours  for  the  possession  of  the  territory  between 
them,  as  in  the  ideal  illustration  of  stream  development 
previously  considered.  The  branches  of  the  larger  master- 
streams,  by  having  their  places  of  discharge  lowered  more 
rapidly  than  adjacent  subsequent  streams  flowing  to  weaker 
consequent  streams,  were  able  to  extend  their  head  branches 
and  capture  new  territory  in  the  manner  already  discussed. 


I;  1 

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198 


RIVERS  OF  NORTH  AMERICA 


This  process  was  modified  in  many  ways,  however,  owing 
to  complex  folding  in  the  rocks  exposed  by  erosion. 

Influence  of  Folds  in  the  Rocks  on  Stream  Adjustment. — A 
fold  in  stratified  rocks  of  various  degrees  of  resistance,  when 
the  axis  is  horizontal,  will  produce  parallel  ridges  and  valleys 
with  tapering  ends.  If  the  axis  of  the  fold  is  not  horizon- 
tal but  inclined  so  as  to  pass  below  a  horizontal  plane  in  one 
direction  and  rise  above  it  in  the  opposite  direction,  it  is 
evident  that  if  the  region  where  the  fold  occurs  is  carved 
away  to  a  horizontal  plane,  and  then  etched  so  as  to  leave 
the  edges  of  the  hard  layers  in  relief,  the  resulting  ridge 
will  not  be  parallel  throughout,  but  form  more  or  less 
elliptical  curves.' 


^^i^^'!f»^i!U^W^fif'^!^:?:^_^*l}^^'..  "ty^a-. 


Synclinal  Fold,  with  Central 
Canoe-Shaped  Valley. 


Anticlinal  Fold,  with  Hemi- 
Cigar-Shaped  Mountain. 


Fig.  19.     Topographic  Forms  Resulting  from  the  Erosion  of  Folded 
Rocks.     (After  Bailey  Willis). 

The  topographic  changes  resulting  from  the  weathering 
and  erosion  of  rocks  of  various  degrees  of  resistance,  when 

'  Folds  in  the  rocks,  if  traced  to  where  they  die  out,  will  be  found  either  to 
flatten  and  spread  so  as  to  merge  with  undisturi>ed  areas  or  become  narrow  and 
mt)re  or  less  sharp-pointed.  Individual  folds  are  more  or  less  conical  and  when 
cut  by  planes  of  erosion  give  figures  which  arc  conic  sections. 


r^vpa'^ 

w 

;^^ 

>        N^'j'^' 

STREAM  DEVELOPMENT 


»99 


folded,  are  shown  in  Fig.  19.  In  one  instance  the  fold  is 
downward,  so  that  the  strata  on  the  borders  dip  toward  the 
longer  axis,  and  is  termed  a  synclinal ;  and  in  the  other 
instances  the  strata  forming  the  arch  dip  away  from  the 
longer  axis,  making  a    anticlinal. 

Water-Gaps  and  JVind-Gaps.— The  Appalachian  Mount- 
ains are  due  to  the  upraising  of  a  great  belt  of  country  in 
which  the  rocks  have  been  folded  into  anticlinal  and  syn- 
clinal, as  in  the  illustration  just  given.  The  longer  axes  of 
the  folds  trend  N.  E.  and  S.  W.,  and  are  either  horizontal 
or  pitch  at  various  angles.  The  western  side  of  each  fold 
is  usually  steeper  than  the  eastern  side.  The  ends  of  the 
folds  frequently  overlap,  one  dying  out  and  another  begin- 
ning and  continuing  sometimes  for  scores  of  miles,  before 
it  in  turn  disappears  and  is  replaced  by  another  similar  fold. 
It  is  the  ridges  in  these  variously  truncated  folds,  due  to 
the  weathering  out  of  resistant  layers,  that  give  to  the 
Appalachians  their  highly  characteristic  topography.  The 
softer  beds  have  been  eroded  away  by  the  subsequent 
streams.  The  ridges  of  resistant  rock  form  the  divides  be- 
tween the  branches  of  the  large  rivers.  The  crests  of  the 
ridges  are  nearly  level  for  the  reason  that  the  region  was 
worn  down  to  a  peneplain  before  the  etching  process  which 
gave  them  prominence  was  initiated.  These  level  crest- 
lines  are  broken,  however,  by  deep  notches  where  the 
master  streams  pass  through  them,  and  are  also  indented 
by  less  deep  notches  where  streams  which  have  been 
beheaded  formerly  crossed  them. 

The  process  of  river  conquest,  as  it  has  been  termed,  by 
which  notches  have  been  left  in  the  crest-lines  of  the  ridges. 


^p 


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Ir 

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t 

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1 

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200 


HI  VERS  OF  NORTH  AMERICA 


is  illustrated  by  the  following  typical  example  in  Virginia, 
borrowed  from  an  admirable  essay  on  the  northern  Appa- 
lachians, by  Willis.' 

The  Potomac  near  Harper's  Ferry  flows  through  two  deep 


THE                            ^^ 

KITTATINNV                  J^-^ 
PLAIN                  ^f   // 

j^fS 

(     ^^-Jy 

/--''^'''^^^ 

^A 

(y^f 

.•^^ 

i^ 

Arrangement  of  Streams  on  the 
Kittatinny  Peneplain. 


Adjusted  Streams  on  the 
Shenandoah  Peneplain. 


Fig.  2o.     Methods  and  Results  of  River  Piracy.     (After  Bailey  Willis.) 

picturesque  notches  in  ridges  of  hard  rock,  as  may  be  seen 
from  the  photograph  forming  Fig.  B,  Plate  XVI.  Such  a 
notch  in  a  mountain  crossing  the  course  of  a  stream,  and  still 
occupied  by  the  stream  which  excavated  it,  is  known  in  the 
language  of  geography  as  a  water-gap.  In  the  Blue  Ridge, 
a  few  miles  south  of  Harper's  Ferry,  there  is  a  similar 
notch,  but  not  so  deep,  the  bottom  of  which,  like  the  crest 

'  Bailey  Willis,  "  The  Northern  Appalachians,"  in  National  Geographic  Mono- 
graphs, vol.  i.,  pp.  169-202,  published  by  the  American  Book  Co.  under  the 
auhpiccs  of  the  National  Geographic  Society. 


STREAM  DEVELOPMENT 


20 1 


of  the  ridge  to  the  north  and  south,  is  a  divide  between  the 
streams  flowing  east  and  those  flowing  west.  The  air-cur- 
rents flow  through  such  notches,  and  this  has  gained  for  them 
the  name  of  wind-gap  among  the  inhabitants  of  the  Appa- 
lachian region.  This  familiar  name  has  been  adopted  by 
geographers  as  a  generic  term  by  which  to  designate  a  class 
of  notches  in  the  crest-lines  of  ridges  and  mountains  having 
a  certain  origin.  The  particular  notch  in  the  Blue  Ridge 
here  referred  to  is  known  as  "  Snickers  Gap,"  and  is  a  typi- 
cal illustration  of  a  wind-gap.  Its  history  is  shown  graphi- 
cally on  the  two  accompanying  sketch-maps  by  Willis. 

It  will  be  remembered  that  the  country  about  Harper's 
F'erry  was  at  one  time  in  the  condition  of  a  tilted  peneplain, 
and  that  a  strong  consequent  stream,  the  Potomac,  flowed 
across  it  toward  the  east ;  and  that  this  river  and  its  tribu- 
taries deepened  their  channels  and  entrenched  themselves  in 
the  plain.  As  a  result  of  this  process  and  of  general  erosion, 
the  edges  of  the  layers  of  hard  rocks  beneath  the  surface 
of  the  plain  were  left  in  relief.  The  Blue  Ridge,  composed 
of  hard,  resistant  quartzite,  is  a  typical  illustration  of  a  ridge 
originating  in  this  manner. 

The  conditions  after  the  Potomac  had  begun  to  deepen 
its  channel,  and  the  Blue  Ridge  was  a  line  of  faint  relief, 
are  shown  in  the  ideal  sketch-map  to  the  left  of  Fig.  20. 
The  infant  Shenandoah  River  entered  the  Potomac  above 
the  water-gap  at  Harper's  Ferry,  flowing  northward  along 
the  west  base  of  the  Blue  Ridge  in  a  valley  excavated  in 
limestone.  At  the  stage  in  the  history  shown  in  the  map 
just  referred  to,  the  Shenandoah  was  a  young  subsequent 
stream.     To  the  south  of  the  Potomac,  as  indicated  on  the 


202 


RIVERS  OF  NORTH  AMERICA 


if: 


M 
<* 


map,  there  was  another  but  weaker  consequent  stream, 
Beaverdam  Creek,  which  also  crossed  the  Blue  Ridge  in  a 
water-gap.  It  will  be  remembered  that  all  of  these  streams 
were  flowing  at  a  higher  level  than  the  Shenandoah  occupies 
to-day,  and  that  the  Blue  Ridge  was  a  much  less  prominent 
topographic  feature  than  at  present.  The  Potomac,  being  a 
larger  stream  than  Beaverdam  Creek,  deepened  its  channel 
more  rapidly,  and  thus  lowered  the  mouth  of  the  Shenandoah 
and  caused  it  to  flow  more  swiftly.  The  Shenandoah  was 
thus  enabled  to  deepen  its  channel  and  to  extend  its  head 
branches  more  rapidly  than  its  neighbour  and  rival.  As  a 
result,  the  Shenandoah  captured  the  drainage  of  Beaverdam 
Creek  to  the  west  of  the  Blue  Ridge,  thus  beheading  that 
stream.  The  ability  of  Beaverdam  Creek  to  deepen  its 
channel  in  the  hard  rocks  forming  the  Blue  Ridge  was  thus 
lessened,  and  as  the  Shenandoah  continued  to  lower  its  chan- 
nel, the  portion  of  Beaverdam  Creek  situated  between  the 
places  of  capture  and  the  bottom  of  the  notch  it  had  cut  in 
the  Blue  Ridge  was  reversed,  and  also  contributed  its  waters 
to  the  capturing  stream.  Beaverdam  Creek  was  thus  broken 
in  two  at  the  place  where  it  formerly  crossed  the  hard  layer 
forming  the  Blue  Ridge,  and  stream  corrasion  there  ceased. 
The  notch  previously  made  was  thus  changed  from  a  water- 
gap  to  a  wind-gap.  Subsequently  the  Blue  Ridge  became 
more  and  more  prominent  owing  to  the  removal  of  the  softer 
rocks  on  each  side  of  it,  but  the  notch  in  its  crest-line  re- 
mained. The  conditions  as  they  exist  at  the  present  day, 
when  the  bottom  of  the  notch  is  a  water-parting  or  divide, 
between  streams  flowing  in  opposite  directions,  is  shown  on 
the  right-hand  map  of  Fig.  20.     Another  ^'.lustration  of  the 


STREAM  DEVELOPMENT 


203 


robbing  of  one  stream  by  a  more  favourably  circumstanced 
rival  is  illustrated  in  Plate  XIII,  and  described  in  advance 
in  connection  with  a  discussion  of  the  migration  of  divides. 

Stream  Conquest. — The  process  of  capture  by  the  subse- 
quent branches  of  strong  consequent  streams,  just  illustrated, 
has  gone  on,  with  various  modifications  in  detail,  through- 
out the  Appalachians,  and  great  variety  both  in  the  direction 
of  the  streams  and  in  the  relief  of  the  land  has  resulted. 
The  principal  conditions  which  give  one  stream  an  advantage 
over  its  neighbour  on  the  opposite  side  of  a  common  divide, 
are  a  shorter  course  to  the  sea,  greater  volume,  and  softer 
rocks  in  which  to  sink  its  channel. 

If  one  of  two  streams  heading  against  a  common  divide 
has  a  shorter  course  to  the  sea  than  its  rival,  other  condi- 
tions being  the  same,  its  gradient  will  be  greater,  and  hence 
it  will  have  greater  velocity  and  be  able  to  corrade  more 
rapidly,  and,  also,  the  amount  of  rock  to  be  removed  in 
order  to  reach  baselevel  is  less.  If  the  rain-fall  on  one  side 
of  a  mountain  range  is  greater  than  on  the  opposite  side, 
other  conditions  being  the  same,  the  streams  on  the  side 
having  the  greater  rain-fall  will  be  larger,  and  hence  able  to 
deepen  their  channels  and  extend  their  head  branches  more 
rapidly  than  the  streams  on  the  opposite  side  of  the  divide. 

In  a  similar  way,  it  will  readily  be  seen  that  of  two  com- 
petiii^y  streams,  one  flowing  over  hard  and  the  other  over 
soft  rocks,  the  one  with  the  easier  task  will  deepen  its  chan- 
nel more  rapidly  than  the  one  flowing  over  hard  rock,  and 
lead  to  the  capture  of  some  of  the  territory  previously 
draining  tu  its  rival. 

In  the  sculpturing  of  the  Appalachian  Mountains  all  of 


TP^ 


fiilil 


Nl! 


m  i 


204 


RIVERS  OF  NORTH  AMERICA 


these  variations  in  conditions,  and  possibly  still  others,  have 
been  in  progress  at  the  same  time.  Many  of  the  features 
of  the  northern  Appalachians  resulting  from  this  process  of 
the  adjustment  of  streams  to  the  structure  of  the  rocks  into 
which  they  sink  their  channels  may  be  read  on  the  map 
forming  Plate  X. 

This  map  illustrates  in  an  admirable  manner  the  way  in 
which  the  Susquehanna,  a  strong  consequent  stream,  flows 
across  the  edges  of  both  hard  and  soft  strata,  independently 
of  the  topography,  while  the  secondary  streams  follow  the 
outcrops  of  the  soft  rocks,  although  occasionally  flowing 
directly  through  a  ridge  of  hard  rock,  and  continuing  their 
general  course  in  the  .lext  valley.  Evidently  the  secondary 
streams,  flowing  through  ridges  of  hard  rock,  had  their  right 
of  way  established  on  the  original  peneplain,  or,  by  a  process 
of  adjustment,  described  below,  have  made  for  themselves  a 
way  out  of  synclinal  valleys  and  into  depressions  excavated 
in  the  tops  of  anticlinal  ridges. 

The  student  should  study  also  the  well-developed  den- 
dritic drainage  in  Lebanon  valley  at  the  bottom  of  the  map, 
where  the  rocks  are  soft  limestone  which  yield  readily  to 
solution  ;  in  comparison  with  the  remarkably  straight  course 
of  Stony  Creek,  confined  between  two  ridges  of  hard  sand- 
stone. The  crooked  courses  of  the  streams,  particularly 
where  they  have  cut  down  their  channel  nearly  to  the  level 
of  the  master  rivers,  indicate  the  tendency  of  streams  well 
advanced  in  their  task  of  valley-making,  to  become  sluggish 
and  meander.  Especially  does  this  excellent  map  illustrate 
the  fact  that  the  relief  of  the  land  is  due  to  the  action  of  the 
stream  in  removing  soft  rocks  and  allowing  the  hard  rocks 


in 


Plate  X. 


Western  Portion  of  the  Anthracite  Basin,  Pennsylvania,  Showing  Canoe-Valleys 
and  Mountains  and  the  Course  of  the  Susquehanna  across  them.  (After 
Bailey  Willis.) 

Approximate  scale :  one  inch  =  twelve  miles. 


^j(K'S 


•0- 


h 


4 
ii 

I 

':  ^V 

1 

1 

i 

mt 


STREAM  DEVELOPMENT 


205 


to  Stand  in  relief.  Instead  of  the  topography  controUing 
the  stream,  the  stream  gives  origin  to  the  topography. 
As  long  since  explained  by  Hutton,  each  stream  flows  in  a 
valley  of  its  own  making. 

Ancient  Peneplains. — Only  a  part  of  the  history  of  the 
northern  Appalachians  has  been  read  when  the  adjustment 
of  the  streams  to  geological  structure  and  the  growth  of 
certain  drainage  areas  at  the  expense  of  others  has  been 
worked  out.  The  land  has  not  remained  stationary  during 
the  millions  of  years  required  for  this  process,  but  have  been 
upheaved  and  depressed.  One  immensely  long  period  of 
rest  is  recorded  by  the  broad,  featureless  peneplain  which 
was  tilted,  as  already  explained,  so  as  to  cause  the  master 
stream  to  flow  across  the  future  site  of  the  mountains  to  the 
sea.  A  portion  of  this  peneplain  not  yet  consumed  forms 
the  Kittatinny  Mountains  in  Eastern  Pennsylvania.  The 
plain  referred  to  is  hence  named  the  Kittatinny  peneplain.' 
After  this  plain  had  been  deeply  dissected  and  the  streams 
had  broadened  their  valleys  so  that  only  isolated  remnants 
of  the  rocks  remained  above  sea-level,  the  region  was  again 
elevated,  but  the  second  upward  movement  was  not  so  great 
as  the  one  just  considered,  and  another  attempt  to  reduce 
the  region  to  baselevel  was  begun.  A  second  peneplain  was 
formed,  but  not  carried  to  completion  before  the  region  was 
again  raised.  As  stated  by  Willis,  the  upheaval  leading  to  the 
dissection  of  the  Kittatinny  peneplain,  caused  an  elevation  of 
two  hundred  feet  in  New  Jersey,  six  hundred  feet  in  Pennsyl- 


'  Also  known  as  the  "  Schooley  peneplain  "  in  New  Jersey.  See  "  Physical 
Geography  of  New  Jersey,"  in  vol.  iv.,  of  the  Final  Report  of  the  State 
Geologist  0/ New  Jersey,  p.  85,  1898. 


r 


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'III 
141 

'J! 
-it 
ill 


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% 


206 


RIVERS  OF  NORTH  AMERICA 


vania,  one  thousand  seven  hundred  feet  in  Southern  Virginia, 
and  thence  southward  decreased  to  the  Gulf  of  Mexico.  A 
typical  illustration  of  this  second  peneplain  is  found  in  the 
broad  bottom  of  the  Shenandoah  valley,  and  for  this  reason 
it  has  been  named  the  Shenandoah  peneplain.  In  the  illus- 
tration forming  Fig.  B,  Plate  XVI,  the  Shenandoah  pene- 
plain is  on  a  level  with  the  point  of  view,  while  the  hills 
rise  to  the  level  of  the  Kittatinny  peneplain. 

In  consequence  of  renewed  elevation  after  the  broadening 
of  the  Shenandoah  peneplain  was  well  under  way,  the 
energy  of  the  streams  was  again  revived.  Once  more  fall- 
ing swiftly,  they  have  sawed  and  are  sawing  their  channels 
down,  and  are  preparing  for  the  development  of  a  future 
baselevel. 

In  the  southern  Appalachians,  the  Kittatinny  peneplain 
was  not  so  completely  developed  as  farther  north,  and  a 
group  of  mountains  representing  the  previous  uplands  from 
which  the  plain  was  carved  was  left.  These  mountains 
have  maintained  their  existence  to  the  present  day,  and  still 
furnish  the  highest  and  most  picturesque  peaks  in  the  sys- 
tem. This  great  group  of  ^^ijaks,  of  which  Mount  Mitchell, 
Rhone  Mountain,  and  other  prominent  summits  in  Eastern 
Tennessee  and  Western  North  Carolina  are  examples,  it  will 
be  seen,  arc  of  the  nature  of  remnants  rising  above  a  broad 
and  nearly  completed  peneplain,  or,  to  use  a  technical  name 
previously  explained,  they  are  monadnocks. 

At  the  south,  the  great  Kittatinny  peneplain  was  not  tilted 
south-eastward  as  in  Eastern  Pennsylvania  and  adjacent 
States,  but  toward    the  south-west.      Hence   the   streams 


riowing  down  its  inclined  surface  had  their  sources  to  the 


STREAM  DEVELOPMENT 


207 


tilted 

jacent 

t  reams 

to  the 


east  of  the  ridges  and  mountains  subsequently  developed 
by  the  erosion  of  its  surface  and  flowed  westward  to  the 
Mississippi  and  south-westward  to  the  Gulf  of  Mexico.  The 
principal  consequent  streams  in  this  region  arc  New  River 
and  the  Tennessee.  The  Coosa  at  present  belongs  also 
in  this  category,  but,  as  has  been  shown  by  Hayes,'  was 
formerly  a  continuation  of  the  Tennessee. 

Synclinal  Mountains  and  Anticlinal  Valleys. — The  process 
of  stream  adjustment  to  geological  conditions,  discovered  as 
erosion  progressed,  was  much  the  same  in  the  southern  as 
in  the  northern  Appalachians,  but  the  details  were  in  some 
respects  different.  One  of  the  characteristic  features  in  the 
structure  of  the  southern  Appalachians,  but  more  especially 
of  their  western  half,  which  departs  somewhat  widely  from 
the  typical  structure  at  the  north,  is  the  presence  of  broad 
downward  curves  in  the  rocks,  or  synclinals,  separated  by 
comparatively  narrow  upward  folds,  or  anticlinals.  If  the 
land  had  been  elevated  without  being  eroded,  we  should 
find  to-day  a  series  of  prominent  but  narrow  ridges  run- 
ning north-east  and  southward,  intervening  between  much 
broader,  trough -shaped  valleys.  If  streams  should  come 
into  existence  on  such  a  surface  where  it  was  inclined  in 
the  direction  of  the  longer  axis  of  the  ridges  and  troughs, 
they  would  evidently  follow  the  depressions  as  consequent 
streams. 

In  marked  contrast  to  what  would  have  been  the  topo- 
graphy of  much  of  Eastern  Tennessee  and  the  northern 
portions  of  Alabama  and  Georgia  had  there  been  no  ero- 

'  C.  VVillard  Hayes.   "  (ieomorphology  of  the  Southern  Appalachians,"  in 
Thf  National  GtOi;raphi(  Mai^atim \  vol.  vi.,  pp.  109-111),  1894. 


¥ 


k  ■ 

r 


208 


RIVERS  OF  NORTH  AMERICA 


sion,  we  find  that  where  the  rid^jes  would  have  been,  there 
are  now  valleys  occupied  by  well-developed  drainage  sys- 
tems, while  the  synclinals,  which  would  have  been  valleys 
under  the  conditions  just  assumed,  in  reality  stand  in  relief 
and  form  broad  ridges  or  mountains,  with  shallow  depres- 
sions in  their  surfaces.  This  reversion  of  what  would  have 
been  the  topographic  relations  of  the  anticlinals  and  syncli- 
nals had  there  been  no  erosion,  is  one  of  the  most  interest- 
ing chapters  in  Appalachian  history.  Let  us  see  how  this 
topographic  revolution  has  come  about. 

A  good  example  of  a  synclinal  plateau  is  furnished  by 
Lookout  Mountain,  which  terminates  at  the  north  in  a  bold 
escapement  over  one  thousand  five  hundred  feet  high  at 
Chattanooga,  Tennessee,  and  extends  south-west  about 
seventy  miles  to  Atalla  and  Gadsden  in  Alabama.  It  was 
at  the  extreme  northern  end  of  this  synclinal,  the  axis  of 
which  declines  gently  southward.,  left  in  bold  relief  by  the 
erosion  of  the  bordering  valleys,  that  the  battle  of  Lookout 
Mountain  was  fought.  To  the  west  of  Lookout  Mountain 
is  a  deep  anticlinal  valley  from  four  to  five  miles  wide,  and 
to  the  west  of  this,  again,  another  broad  synclinal  plateau 
known  as  Sand  Mountain.  The  present  relief  and  drainage 
of  this  region  are  shown  on  the  sketch-map  forming  Fig.  4, 
Plate  XL 

The  origin  of  the  present  strongly  pronounced  and  char- 
acteristic topography  of  the  region  just  referred  to,  has  been 
studied  by  Ilay.s,  and  I  cannot  serve  the  reader  better 
than  by  presenting  an* extract  from  his  report.' 


'  Geological  Survey  of  Alabama,  Billetin,  No.  4,  pp.  23-29,  and  Plate  I 
189a. 


STREAM  DEVELOPMENT 


209 


The  four  maps  presented  on  Plate  XI.  illustrate  four  suc- 
cessive stages  in  the  history  of  the  streams  in  the  Lookout- 
Sand  mountain  region.  An  early  stage  in  the  topographic 
development  is  shown  in  Fig.  i,  where  a  stream  flowed 
southward  in  each  of  the  synclinal  troughs  and  received 
tributaries  from  the  adjacent  anticlinal  ridges. 


"  At  first  these  streams  were  all  flowing  upon  the  same  kind  of 
rock,  probably  coarse  sandstone,  so  that  they  were  able  to  erode 
their  channels  most  rapidly  where  the  fall  and  consequently  the 
transporting  power  of  the  stream  was  the  greatest.  But  the  slope 
of  the  synclinal  troughs  was  very  slight,  so  that  the  main  streams 
had  little  power  to  deepen  their  channels,  while  the  side  streams 
flowing  into  them  from  the  intervening  ridge,  although  they  were 
much  smaller,  still  by  reason  of  their  greater  fall  eroded  their 
beds  more  rar'dly. 

"  If  the  rocks  of  this  region  had  been  uniform  in  character  for 
a  long  distance  down  from  the  surface,  the  effect  of  the  more 
rapid  cutting  of  the  side  streams  would  have  been  simply  to  re- 
duce the  height  of  the  intervening  ridge,  leaving  the  main  streams 
in  their  original  position  in  the  synclinal  troughs.  But  the  rocks 
are  not  homogeneous.  They  consist  of  alternating  hard  and  soft 
beds,  and  after  the  side  streams  had  cut  down  a  few  hundred  feet 
they  came  to  layers  of  shale  and  then  limestone  whi(  h  they  could 
remove  much  tnore  rapidly  than  the  overlying  sandstone.  As 
soon  as  a  side  hiream  reached  these  soft  rocks  at  any  point,  it 
tciuled  to  widen  its  valley  at  that  point  by  removing  the  soft  rocks 
so  .  to  undermine  and  thus  break  down  the  overlying  harder 
beds  By  a  continuation  of  this  process  lateral  valleys  were 
formt  ,  Fig.  2  [Plate  XI],  extending  in  the  direction  of  the  ridge 
and  at  right  angles  to  the  side  streams. 

"  The  two  streams  w  and  w'  cut  away  at  the  divide  d  and 

the  stream  w  having  the  lowest  outlet  was  able  to  erode  more 

rapidly  than  w   and  so  pushed  the  divide   farther  and  farther 

toward  the  side  stream  /'  till  finally  it  tapped  the  latter  and  led 

•4 


I 


EXPLANATION  OF  PLATE  XL 

Sketch  map  of  a  portion  of  Lookout  Mountain,  Wills  Valley,  and  Sand 
Mountain  ;  showing  various  stages  in  the  development  of  the  present  drainage 
system  and  topography. 

Fig.  I.  Showing  the  undulating  surface  which  determined  the  initial  posi- 
tion of  the  streams.  B,  B  and  T,  T  flow  in  synclinal  troughs  and  receive  side 
streams  1,  1,  1  and  s,  s,  s  from  the  intervening  anticlinal  ridge. 

Fig.  2.  Showing  a  stage  in  the  development  when  the  side  streams  1,  1,  1 
and  s,  s,  s  have  cut  through  the  hard  surface  rocks  to  soft  beds  beneath  and 
lateral  valleys,  w,  w',  etc.,  are  being  formed  paj.illel  with  the  anticlinal  axis. 

Fig.  3.  Showing  a  further  stage  in  the  development  when  the  lateral 
stream  w  has  cut  through  the  intervening  divides  and  diverted  the  drainage 
of  V  and  1".  A  second  series  of  lateral  streams,  W,  etc.,  is  being  developed 
along  the  anticlinal  parallel  with  the  first  series,  W,  W,  etc.  The  upper  portion 
of  the  synclinal  stroam,  B,  B  has  been  diverted  by  a  side  stream,  R,  R. 

Fig.  4.  Showing  the  present  drainage  system  and  topography.  Both  lateral 
streams,  w  and  W,  have  continued  to  encroach  upon  the  basins  of  those  adja- 
cent, w  resulting  in  the  present  Little  Wills  Creek  and  W  in  Big  Wills  Creek. 
The  latter  now  occupies  the  axis  of  the  anticlinal  throughout  its  whole  length 
and  has  become  the  dominant  stream  of  the  system  under  consideration.  The 
stream  B,  B  has  been  further  robbed  of  portions  of  its  drainage  basin  by  streams 
flowing  eastward  to  the  Coosa.  The  synclinal  stream  T,  T  has  been  tapped  at 
various  points  by  streams  flowing  westv.artl  into  the  Tennessee  and  the  drain- 
age thus  diverted  from  its  original  course  toward  the  south. — C.  W.  IIavks. 


i 
I 


Plate  XI. 


.  s 


L(K)kout  Mountain  Region,  Alabama,  illustrating  stream  adjustment. 
(After  E.  W.  Hayes). 

Approximate  scale :  i  inch  ~  15  milM. 


wm 


M 
II 


r 


* 


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f 

VI 

1 

*'i' 


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5  TREA  M  DE  VEL  0PM EN  T 


211 


its  waters  off  by  way  of  /.  The  lateral  stream  thus  formed  {w  Wy 
Fig.  3)  was  the  beginning  of  the  present  Little  Wills  Creek.  The 
same  process  would  have  continued  till  this  stream  had  tapped 
successively  all  the  drainage  basins  above  if  it  had  not  en- 
countered a  second  hard  bed  through  which  erosion  was  very 
slow.  After  the  soft  rocks  were  removed  which  lay  above  this 
nard  bed,  then  the  same  process  was  repeated  which  had  taken 
place  on  the  original  anticlinal  ridge;  that  is,  side  streams  with 
greater  fall  were  able  to  cut  through  the  hard  bed  more  quickly 
than  the  main  stream  and  reaching  soft  rocks  belov/  began  to 
widen  their  valleys  and  then  form  lateral  valleys  at  these  points. 
The  same  process  was  repeated  with  this  second  set  of  lateral 
streams.  By  erosion  at  the  divides  D  D'  D*,  etc,  P'ig.  j,  that 
one  having  the  lowest  outlet  tapped  the  drainage  basin  of  the  one 
adjacent  and  led  its  waters  off  along  the  axis  of  the  anticlinal. 
The  side  stream  W,  Fig,  3,  possessed  a  great  advantage  over 
any  of  the  others  in  having  a  much  lower  outlet  and  hence  it  was 
able  to  encroach  upon  them  and  divert  their  head-waters  to  its  own 
channel.  But  with  each  conquest  of  new  territory,  by  the  addi- 
tional volume  of  water  thus  gained,  it  became  more  efficient  in 
eroding  its  valley  while  the  streams  whose  drainage  basins  had 
been  thus  diminished  were  even  less  able  to  hold  their  own. 
Thus  the  process  was  cumulative  in  its  effects,  and  finally  the 
stream  last  formed  became  the  dominant  one  of  the  drainage 
system. 

"  This  process  by  which  the  ridge  was  removed  and  the  streams 
shifted  from  their  original  position  in  the  synclinal  trough  to  the 
axis  of  the  anticlinal  may  be  further  illustrated  by  the  diagram 
[Fig.  21],  representing  a  section  through  the  ridge  and  adjacent 
troughs.  The  hea^'y  line  represents  the  present  surface  and  the 
unbroken  lines  the  beds  of  rock  as  they  exist  at  present,  below  the 
surface.  The  curved,  dotted  lines  represent  the  position  of  the 
beds  as  they  originally  existed  before  their  removal.  Two  of  these 
beds,  CI  and  Sr,  which  are  sandstone,  offer  much  greater  re;>istance 
to  erosion  than  the  limestones,  Cb,  Sc,  and  Sk,  The  upper  curved 
line  represents  the  profile  of  the  land  surface  on  which  drainage 
originated,  corresponding  in  position  to  the  line  M  N  in  Fig.  i 


/.■v-'t;' 


212 


RIVERS  OF  NORTH  AMERICA 


f. 
I 


[Plate  XI.].  B  and  T  indicate  the  position  of  the  main  streams  in 
the  synclinal  troughs  into  which  the  side  streams  flowed  fiom  the 
intervening  anticlinal  ridge.  As  already  explained,  the  first 
point  at  which  the  upper  hard  bed  C  1  was  cut  through  was  on 
the  steep  slopes  of  this  ridge,  as  at  w  and  v,  where  the  lateral 
valleys  were  subsequently  formed  in  a  direction  parallel  with  the 
ridge.  The  upper  broken  line,  then,  will  represent  the  surface 
at  the  second  stage  of  its  development,  represented  in  Fig.  2. 
Continuing  to  erode  their  channels  downward  through  the  soft 
rocks  Cb,  the  streams  encountered  the  second  hard  bed  Sr,  and 


Fig.  21.     Section  through  Lookout  Mountain,  Wills  Valley,  and  Sand  Mount- 
ain ;  Showing  Profile  of  the  Land  Surface  at  Four  Stages  in  the  Develop- 
ment of  the  Present  Drainage  System.     (After  C.  W.  Hayes.) 

the  process  above  described  was  repeated,  producing  the  surface 
represented  by  the  lower  broken  line.  This  is  the  third  stage  in 
the  development  of  the  drainage  system  in  which  the  streams  had 
the  positions  indicated  in  Fig.  3.  Finally,  the  lateral  stream 
which  started  at  W,  being  already  through  the  two  hard  beds, 
easily  distanced  its  competitors  and  robbed  them  of  successive 
portions  of  their  drainage  area  until  it  became  the  dominant 
stream  of  the  system.  Big  Wills  Creek. 

"  The  stream  B  B  Fig.  i,  which  originally  flowed  the  whole 
length  of  the  Lookout  synclinal  trough,  was  not  permitted  to  re- 
tain that  position.  Robbed  of  all  its  western  tributaries  by  the 
process  above  described,  it  was  unable  with  its  diminished  volume 
to  lower  its  channel  sufficiently  fast  for  its  own  protection.  At  a 
point  on  the  eastern  side  of  the  synclinal  a  stream,  R  R,  Fig.  3, 
has  cut  back  from  the  valley  of  the  Coosa  and  diverted  the  upper 
portion  from  its  original  channel.  Hence  the  present  course  of 
Little  River,  Fig.  4,  follows  the  synclinal  to  this  point  and  then 
turns  sharply  to  the  south-east  by  a  deep  rocky  gorge.     Other 


STREAM  DEVELOPMENT 


213 


portions  have  been  more  recently  diverted  by  Wolf  and  Yellow 
Creeks.  Thus  the  stream  which  originally  drained  the  whole  of 
the  synclinal  trough  and  the  slopes  of  the  adjacent  anticlinal 
ridges  has  been  robbed  of  the  greater  part  of  its  drainage  basin, 
and  Black  Creek  alone,  a  mere  remnant  of  the  original  stream, 
retains  the  course  which  it  has  followed  from  the  beginning. 
For  a  short  time  during  the  stage  represented  by  Fig.  3,  Little 
Wills  Creek,  w  w,  was  the  encroaching  stream,  but  reaching 
a  hard  stratum,  its  career  of  conquest  was  checked,  and  the 
stream  W,  more  favourably  situated,  although  last  born,  be- 
came the  dominant  stream  of  the  system.  In  the  meantime  the 
anticlinal  ridge  had  been  entirely  removed,  and  in  its  place 
a  deep  valley  excavated,  while  the  original  stream  channels 
were  left  high  up  in  the  synclinals  now  forming  the  tops  of  the 
mountains. 

"  This  is  but  one  of  numerous  examples  in  this  region  which 
might  be  followed  out  in  detail  to  show  how  the  drainage  system 
has  adjusted  itself  to  the  structural  surface  and  how  the  present 
position  of  the  streams  is  dependent  on  the  dip  of  the  strata  and 
the  alternation  of  hard  and  soft  rocks. 

"  It  must  be  borne  in  mind,  however,  that  only  the  latest  stages 
in  the  development  of  the  present  topography  can  be  followed 
with  certainty.  The  condition  which  immediately  preceded  the 
present  is  easily  inferred,  but  as  the  processes  are  followed  back- 
ward they  become  more  obscure,  and  finally  are  only  to  be  con- 
jectured. Hence  in  the  development  of  Wills  Valley,  as  it  has 
been  sketched  above,  the  explanation  becomes  more  largely  pure 
hypothesis  as  the  more  remote  stages  are  reached.  The  explana- 
tion offered  for  the  earliest  stages  is  not  the  only  one  possible, 
but  is  perhaps  the  most  probable  of  a  number  which  might  be 
given. 

"  Also  in  the  above  sketch  several  complicating  factors  have 
been  purposely  omitted  for  the  sake  of  greater  simplicity  in  pre- 
senting the  essential  points  of  the  theory.  Thus  the  anticlinal 
arch  probably  continued  to  rise  during  the  process  of  erosion, 
and  the  effect  which  this  may  have  had  on  the  resulting  topography 
has  not  been  taken  into  consideration.'" 


1 

\ 

;i4 


RIVERS  OF  NORTH  AMERICA 


The  explanation  accompanying  Plate  XI,  also  borrowed 
from  Hayes's  report,  will  assist  the  reader  in  understanding 
the  bit  of  ancient  history  just  outlined. 

An  analysis  of  SiTeam  adjustment,  in  a  region  of  folded 
rocks  similar  to  that  just  presented,  was  published  in  1889, 
by  Davis '  in  discuiising  the  origin  of  the  peculiar  topo- 
graphy of  Pennsylvania.  The  paper  just  referred  to,  the 
first  ever  published  that  offered  a  rational  explanation  of 
the  development  of  the  intricate  Appalachian  drainage, 
should  be  read  by  all  students  of  the  events  in  the  earth 
history  here  discussed. 


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EFFECTS   OF   ELEVATION   AND    SUBSIDENCE   ON   STREAM 

DEVELOPMENT 

The  effect  of  a  general  rise  of  the  land  throughout  a  broad 
region  is  to  increase  the  gradients  of  the  streams,  and  hence 
to  give  them  greater  velocity  and  greater  corrading  power. 
If  subterranean  forces  affect  the  surface  in  such  a  manner  as 
to  tilt  a  broad  area,  the  streams  flowing  in  the  direction  of 
downward  tilting,  other  conditions  remaining  unchanged, 
will  have  their  energy  increased,  and  will  therefore  deepen 
their  channels.  This  will  give  their  lateral  tributaries, 
coming  to  them  more  or  less  nearly  at  right  angles,  increased 
energy  by  lowering  their  mouths,  and  still  other  results  will 
follow. 

The  reader,  no  doubt,  has  already  reached  the  con- 
clusion that  a  river  system  is  similar  to  a  delicately  ad- 
justed machine.     A  change  in  the  adjustment  in  any  one 

'  W.  M.  Davis,  "  The  Rivers  and  Valleys  of  Penn!>ylvaiiia,"  in  the  National 
Geographic  Magazine,  vol.  i.,  pp.  183-253. 


I'l 


STREAM  DEVELOPMENT 


215 


part,  or  in  the  complex  and  far-reaching  conditions  on  which 
stream  life  depends,  necessitates  changes  throughout  large 
portions  and  perhaps  the  whole  of  the  system.  Of  the 
changes  which  interfere  with  the  regular  and  systematic 
development  of  streams,  none  are  more  common  or  have 
produced  more  conspicuous  results  than  movements  in  the 
rocks  resulting  in  upheavals  or  depressions  of  the  surface. 

Some  of  the  effects  of  uplift  and  subsidence  of  portions  of 
the  earth's  crust  have  already  been  considered  in  connection 
with  the  discussion  of  the  origin  of  stream  terraces,  and  of 
the  adjustment  of  stream  to  geological  conditions.  There 
are  many  other  results  of  such  changes,  however,  which  are 
of  interest  to  the  geographer  and  geologist. 

Some  of  the  Effects  of  Elevation. — A  rise  of  a  region  ad- 
jacent to  a  shallow  sea  would  bring  a  portion  of  the  sea- 
floor  above  water,  and  thus  increase  the  area  of  dry  land. 
The  length  of  the  streams  entering  the  sea  in  the  area  thus 
affected  would  be  increased,  and  necessitate  a  readjustment 
of  grade  for  a  long  distance  up  their  courses.  The  lengthen- 
ing of  a  river  by  the  addition  of  a  broad  strip  of  new  land  to 
the  border  of  a  continent  might  necessitate  such  a  readjust- 
ment of  grade  by  deposition,  in  order  to  enable  the  stream 
to  carry  its  burden  to  the  sea,  that  much  of  the  increase  in 
energy  due  to  the  upheaval  would  be  counteracted  so  as  to 
cau^e  the  stream  to  aggrade  its  channel  at  localities  where 
corrasion  was  previously  in  progress.  Elevation,  it  may 
thus  be  shown,  is  not  always  immediately  followed  by 
deeper  cutting,  as  might  at  first  be  inferred. 

An  elevation  of  the  land  of  the  nature  just  considered, 
tends  in  general  to  straighten  coast-lines,  for  the  sea  bot- 


14 


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2l6 


RIVERS  OF  NORTH  AMERICA 


torn,  as  a  rule,  is  more  even  than  land  areas,  and  to  trans- 
form previous  bays  and  estuaries  into  river  valleys. 
;  Increased  complexity  in  the  influence  that  elevation  has 
on  stream  development  occurs  v/hen,  instead  of  a  tilting  of 
broad  areas  along  a  single  axis,  the  rocks  are  folded,  as  in  the 
early  history  of  the  Appalachian  Mountains,  so  as  to  produce 
many  lines  of  elevation.  In  certain  regions,  especially  in 
the  central  portions  of  continental  areas,  the  rocks  have 
been  elevated  into  vast  domes  by  forces  acting  from  below 
upward.  Examples  of  such  domes,  and  of  their  influence  on 
drainage,  are  furnished  by  the  Black  Hills  of  Dakota  and 
other  similar  uplifts  in  the  eastern  part  of  the  Rocky 
Mountain  region. 

Again,  throughout  broad  belts  of  the  earth's  crust  the 
rocks  have  been  broken  by  lines  of  fracture,  and  the  blocks 
thus  formed  variously  tilted  and  displaced.  The  displace- 
ment of  the  broken  edges  of  the  same  bed,  on  the  opposite 
sides  of  such  a  break,  is  in  many  instances  thousands  and 
even  tens  of  thousands  of  feet.  The  Great  Basin  region 
illustrates,  more  clearly  than  any  other  portion  of  North 
America,  the  influence  of  such  faults,  as  they  are  termed, 
on  the  relief  of  the  surface. 

The  changes  in  topography  produced  by  faulting  probably 
progress  slowly  with  many  intermittent  movements.  If 
the  growth  of  the  faults  is  so  slow  that  streams  flowing 
across  the  affected  region  can  deepen,  or  aggrade,  their 
channels  as  rapidly  as  the  changes  occur,  the  streams  may 
hold  their  right  of  way  and  deepen  or  fill  their  channels  as 
rapidly  as  the  land  is  elevated  or  depressed.  When,  for 
example,  a  fault  or  fold  rises  athwart  the  course  of  a  river. 


STREAM  DEVELOPMENT 


217 


it  may  corrade  its  channel  as  rapidly  as  the  land  rises  and 
excavate  a  trench  through  the  growing  mountain.  Illustra- 
tions of  such  a  history  are  furnished  by  the  Columbia  and 
many  of  its  branches,  which  cut  through  the  upturned  edges 
of  fault  blocks  and  produce  water-gaps. 

Should  t'le  growth  of  a  mountain  formed  either  by  a 
folding  of  the  earth's  crust,  by  the  elevation  of  a  dome,  or 
the  growth  of  an  escarpment  due  to  faulting  athwart  the 
course  of  a  river,  be  more  rapid  than  it  can  deepen  its 
channel,  evidently  it  will  be  broken  in  two.  Its  lower 
course  will  be  beheaded,  its  upper  course  reversed,  or  its 
waters  ponded  so  as  to  form  a  lake. 

When  the  normal  development  of  a  drainage  system  is 
interrupted  by  such  changes  as  have  just  been  instanced,  a 
new  adjustment  is  made,  and  the  process  of  development 
continued  under  the  changed  condition. 

The  broad  conclusion  reached  in  reference  to  the  effects 
of  upheaval  on  drainage  is,  that  land  is  raised  above  the 
sea  by  movements  in  the  earth's  crust,  due  principally  to 
the  cooling  and  consequent  shrinking  of  the  earth's  hot 
interior,  and  the  adjustment  of  the  cooler  and  more  rigid 
crust  to  keep  in  contact  with  the  continually  shrinking 
central  mass  on  which  it  rests;  and  streams  and  other  de- 
nuding agencies  cut  away  the  areas  thus  raised.  There 
is  a  continued  warfare  between  these  two  contending  series 
of  agencies. 

Some  of  the  Effects  of  Subsidence. — One  of  the  most  con- 
spicuous effects  of  a  downward  movement  of  the  land  on 
the  streams  draining  it  and  on  the  valleys  that  the  streams 
have  made,  is  to  be  seen  when  coastal  plains  crossed  by 


id'n£<Ik& 

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218 


RIVERS  OF  NORTH  AMERICA 


large  rivers  are  submerged.  In  such  instances  the  sea  enters 
the  river  valleys  and  converts  them  into  bays  and  estuaries. 
Ridges  between  adjacent  valleys  then  become  capes  or  pro- 
montories, and  the  more  isolated  peaks  are  perhaps  con- 
verted into  islands.  Examples  of  conspicuous  changes  due 
to  subsidence  are  furnished  by  the  Atlantic  and  St.  Law- 
rence drainage  slopes.  The  mouth  of  the  Hudson,  as  is 
well  known,  was  formerly  some  seventy  miles  eastward  of 
Long  Island,  but  now,  owing  to  a  depression  of  the  land, 
the  tide  rises  and  falls  at  Troy.  The  St.  Lawrence  formerly 
discharged  to  the  eastward  of  what  is  now  Nova  Scotia, 
but  at  present  the  trunk  of  the  river  is  shorter  by  a  thou- 
sand miles,  ai.d  tide-water  reaches  nearly  to  Monti  eal. 

The  St.  Lawrence  below  Montreal  and  the  Hudson  (Plate 
XV.)  below  Troy  illustrate  what  is  termed  a  drowned  river. 
The  former  rivers  have  been  betrunked  by  subsidence. 
Other  illustrations  of  the  same  occurrence  are  furnished  by 
Delaware  and  Chesapeake  Bays,  which  are  estuaries  formed 
by  the  drowning  of  river  valleys.  Chesapeake  Bay  is  also  a 
typical  example  of  the  way  in  which  a  river  system  may  be 
dissected  by  subsidence.  The  trunk  of  the  stream  has  been 
lost  by  drowning,  and  several  of  the  former  branches  enter 
independent  estuaries. 

Many  other  illustrations  of  similar  geographical  changes 
are  f"-nished  by  the  coast-lines  of  North  America.  The 
ragged  coast  of  Maine,  with  its  multitudes  of  capes  and 
bays  and  its  fringe  of  islands,  owes  much  of  Its  picturesque- 
ness  to  the  fact  that  a  rough  land-surface  has  been  depressed 
so  as  to  allow  the  sea  to  encroach  upon  it.  In  this  in- 
stance, however,  the  land  was  formerly  covered  by  glacial 


STREAM  DEVELOPMENT 


219 


ice,  and  in  part  the  roughness  of  its  surface  is  due  to  hills 
and  ridges  produced  by  glacial  deposition. 

On  the  Pacific  coast 
a  partially  drowned 
river-valley  furnishes 
the  magnificent  bay 
opening  to  the  sea 
through  the  Goidei. 
Gate.  Puget  Sound 
and  the  fringe  of  isl- 
ands adjacent  to  an 
extremely  irregular 
coast,  northward  to 
Lynn  Canal  and  Gla- 
cier Bay,  Alaska,  al- 
so show  the  effect  of 
the  sea  entering  the 
depressions  on  the 
border  of  a  continent 
and  leaving  the  hills 
and  mountains  rising 
above  a  certain  level 
exposed  to  the  air. 
Here   again,    as    on 


the  Maine    coast,  the  F"-  2a.— Map   of  Chesapeake   Bay.  Showint;  hy 

Heavy  Lines  the  Way  in  which  Various  Streams 

roughness  of  the  land  ^^^,,^1  y^jt^  ^^  y^^^  ^  yjngie  r,unk-stream  if 
is     due     in     part      to      the  Land  were  Elevated.    (After  R.  S.   Tarr.) 

glacial  action.  Puget  Sound  and  the  similar  depression 
northward  for  a  thousand  miles  were  formerly  occupied  by 
glacial  ice,  which  lingered  in  the  depressions,  -^nd  allowed 


^^3W 


220 


RIVERS  OF  NORTH  AMERICA 


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'^4 


deep  accumulations  of  stream  and  glacial-born  debris  to  be 
deposited  around  it.  When  the  ice  finally  melted,  the  pres- 
ent tide-ways  were  left  unfilled. 

In  general,  land  exposed  to  the  atmosphere  is  rendered 
rough  and  uneven  by  stream  corrasion  up  to  r  certain  stage 
in  its  topographical  development,  and  then  the  p  '^-minences 
are  removed,  the  relief  becomes  subdued,  and  a  plain  is  the 
ultimate  result.  If  partial  submergence  occurs  during  the 
earlier  stages  of  topographical  development,  a  ragged  co".st- 
line  results.  Beneath  the  sea-level  ^he  detritus  washed 
from  the  land,  together  with  shells  and  other  material  of 
organic  origin,  is  deposited,  and  in  qualities  in  the  bottom 
filled.  A  rise  of  the  sea  bottom,  therefore,  as  previously 
stated,  tends  to  produce     vcn  coast-lines. 

The  action  of  the  waves  and  currents  of  the  ocean  on  its 
shores  is  analogous  in  many  ways  to  the  action  of  rivers  on 
the  bottom  and  sides  of  the  valleys  through  which  they 
ficw,  and  analoi^ous  topographic  changes  result,  which  arc 
of  great  scenic  as  well  as  geographic  importance.  Another 
branch  of  geographic  study  full  of  interest  and  novelty,  is 
here  open  to  the  student,  but  at  present  we  must  deny  our- 
selves the  pleasure  of  a  stroll  on  the  salt-sea  strands  and 
return  to  the  co;isideration  ot  river  development. 

The  topographic  changes  resulting  from  the  drowning  of 
a  river  valley,  owing  to  a  subsidence  of  the  land  adjacent  to 
the  ocean,  find  a  counterpart  along  the  shores  of  lakes 
when  their  wate  s  rise,  in  the  ca«'e  of  lakes  without  out- 
lets, as  Great  Salt  Lake,  Utah  ;  Pyramid,  Winnemucca,  and 
Walker  Lrkes,  Nevada,  etc.,  a  climatic  change,  resulting  in 
an  increased  rain-fall  or  a  decrease  in  evaporation,  would 


STREAM  DEVELOPMENT 


221 


»n.^  '. 


cause  the  lakes  to  rise  and  flood  the  lower  portions  of  low- 
grade  tribute  ry  stream-valleys.  The  extension  of  the 
waters  of  Great  Salt  Lake  into  the  channel  of  Bear  River  at 
the  present  time  is  an  illustration  of  the  drowning  of  a  river 
by  this  process.  The  many  changes  of  level  experienced  hy 
the  Laurentian  lakes,  owing  to  a  tilting  of  their  basins,  has  in 
some  instances  led  to  the  submergence  of  river  valleys.  For 
example,  at  a  time  when  the  waters  in  the  western  part  of 
Lake  Ontario  were  lower  than  now,  Niagara  River  exca- 
vated a  channel  leading  past  Lewiston,  which,  owing  to  a 
tilting  of  the  lake  basin,  was  subsequently  flooded.  Other 
geographical  changes,  produced  by  fluctuation  of  vvat^r-level 
in  the  lakes  referred  to,  illustrate  on  a  comparatively  small 
scale  the  modifications  in  coast-lines  which  result  from 
movement  in  the  land. 

.SOME    OF    THE    INFLUENCES    OF    VOLCANIC    AGENCIES   ON 

STREAM   DEVELOPMENT 

Important  modifications  in  stream  development  have  re- 
sulted in  numerous  instances  from  interruptions  due  to 
volcanic  agencies.  The  influences  of  volcanoes  on  the 
histories  of  streams  are  diverge,  but  only  a  few  of  the  more 
pronounced  changes  of  this  nature  need  be  noted  at  this 
time  to  enable  the  reader  to  recognise  others  without  the 
aid  of  suggestions. 

Volcanoes  frequently  emit  streams  lA  lava  which  flow 
over  the  surface  of  the  land  in  obedience  to  gravity,  and 
take  the  most  favourable  courses  available.  A  lava  stieam 
sv>metimes  flows  dawn  a  previously  eroded  valley,  and  on 
cooling  and  hardening  leaves  it  partially  or  wholly  filled 


i! 


Il 


^1 


222 


RIVERS  OF  NORTH  AMERICA 


» 
r 

I 


with  resistant  rock.  The  adjustment  of  the  stream  is  thus 
greatly  disturbed,  or  perhaps  an  entirely  new  start  in  chan- 
nel-making initiated.  Lava  streams  sometimes  cross  river 
valleys  and  dam  them  so  as  to  form  lakes.  Instances  of 
such  a  nature  are  described  in  the  author's  book  entitled 
Volcanoes  of  North  A  merica. 

Vast  lava  inundations  have  occurred  in  the  history  of 
North  America,  as  when  the  Columbia  lava  of  Washington, 
Oregon,  Idaho,  and  California  was  spread  out  in  successive 
sheets  over  an  area  of  between  two  hundred  thousand  and 
two  hundred  and  fifty  thousand  square  miles  and  attained  a 
thickness  in  certain  regions  of  over  four  thousand  feet.  In 
such  an  instance,  the  topography  of  the  inundated  region  is 
entirely  obliterated,  the  lives  of  its  streams  are  terminated, 
and  a  new  start  in  stream  development  has  to  be  made  on 
the  surface  of  the  lava  covering.  The  streams  from  neigh- 
bouring mountains  are  dammed,  lakes  are  formed  which  dis- 
charge perhaps  across  the  lava  plain,  as  in  the  instance  of 
Snake  River,  and  a  canyon  is  carved.  In  the  case  of  the  river 
just  mentioned,  the  outspreading  of  a  vast  lava  plain  over  the 
region  across  which  it  previously  flowed,  was  in  a  measure 
equivalent  to  an  elevation  of  the  land  in  its  lower  course. 

Volcanoes  also  discharge  projectiles  in  the  form  of  scoria, 
bombs,  small  rock  fragments  {lapilli),  and  dust,  which  ac- 
cumulate about  the  vents  from  which  they  came  and  build 
up  elevationr,  usually  conical  in  form,  or  are  carried  away 
by  the  wind,  as  in  the  case  of  volcanic  dust,  and  widely  dis- 
tributed. Deposits  of  this  nature  also  modify  the  normal 
development  of  streams,  and  may  even  terminate  their 
existence. 


STREAM  DEVELOPMENT 


223 


I  is  thus 
in  chan- 
)ss  river 
inces  of 
entitled 

itory  of 
lington. 
ccessive 
ind  and 
taincd  a 
jet.  In 
■egion  is 
ninated, 
Tiade  on 
n  neigh- 
lich  dis- 
tance of 
:he  river 
over  the 
measure 
3urse. 
f  scoria, 
lich  ac- 
id build 
:d  awuv 
ely  dis- 
normal 
te  their 


Volcanic  eruptions  are  frequently  accompanied  by  earth- 
quakes, which  cause  fissures  to  open  in  the  soil  and  rocks, 
and  in  this  manner  other  accidents,  as  they  may  be  termed, 
to  river  development  come  about.  Earthquakes  are  also 
accompanied  by  changes  of  the  level  of  the  land,  perhaps 
as  a  cau£3  or  as  an  effect  of  the  changes  to  which  the  shocks 
are  due,  but  the  influence  of  such  movements  in  stream  de- 
velopment is  not  essentially  different  from  that  produced 
by  the  changes  in  elevation  already  considered. 

Another  method  by  which  volcanoes  influence  the  lives 
of  streams  is  through  the  action  of  the  acids  they  emit. 
This  chemical  phase  of  the  subject  has  received  but  slight 
attention,  but  it  is  evident,  from  the  vast  quantities  of 
various  acid  gases  poured  out  during  eruptions  and  even 
for  centuries  after  a  volcano  has  pa'^sed  to  the  condition  of 
a  fumarole  or  a  solfatara,  that  the  chemical  action  of  surface 
waters  must  thereby  be  enhanced. 

Volcanoes  also  exert  an  influei  >:e  on  climate  both  on  ac- 
count of  the  heat  emitted  and  the  vapours  poured  into  the 
air,  and  again,  by  reason  of  their  influence  on  air  currents, 
etc.,  when  prominent  mountains  are  built.  All  of  these 
changes  in  what  may  be  termed  the  geography  of  the  air, 
as  well  as  of  the  re-  A  of  ti.e  land,  exert  an  influence  on  the 
history  of  streams. 

SOME  OF  THE   MODIFICATIONS   IN   STREAM    DEVELOPMENT 
DUE  TO   CLIMATIC   CHANGES 

To  appreciate  the  effects  of  climatic  changes  on  normal 
stream  development,  perhaps  the  best  method  is  to  hav*.  in 


mf^'^^ 


i'!wmM>li 


i 


I 


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4 


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I'l 


^    '      J" 


224 


RIVERS   OF  NORTh    AMERICA 


mind  an  example  of  a  river  with  numerous  branches  which 
has  become  well  adjusted  to  the  structure  of  the  rocks 
underlying  its  hydrographic  basin,  and  then  postulate 
atmospheric  changes  and  see  what  variations  in  the  be- 
haviour of  the  stream  would  necessarily  result.  The  con- 
clusions reached  by  this  process  of  mental  analysis  may 
be  checked  by  making  comparisons  between  the  changes 
thought  to  logically  follow  certain  climatic  variations  and 
the  condition  of  the  streams  in  regions  where  similar 
changes  have  actually  occurred. 

Of  the  elements  in  the  highly  complex  atmospheric  con- 
ditions embraced  in  the  term  climate,  those  having  the 
most  direct  and  tangible  influence  on  the  behaviour  of 
streams  are  precipitation  and  temperature. 

Variations  in  Precipitation. — An  increase  in  precipitation 
in  a  region  where  the  mean  annual  temperature  is  such  that 
all  of  the  water  coming  to  the  earth  is  in  the  form  of  rain, 
or  if  snow  falls  during  a  part  of  the  year  it  is  all  melted  be- 
fore the  next  succeeding  winter,  is  to  increase  the  volumes 
of  the  streams  and  enable  them  to  carry  on  their  work  more 
rapidly. 

An  increase  in  rain-fall,  other  conditions  remaining  the 
same,  means,  then,  that  more  debris  is  transported,  and 
when  the  relation  of  load  to  velocity  is  such  as  to  favour 
corrasion,  the  stream  channels  are  deepened  more  rapidly 
than  previous  to  the  postulated  change.  Increased  corra- 
sion in  the  branchea  of  ft  Urninage  system,  however,  means 
usually  more  rapid  depositluM,  u|  a||||m<4in|i  in  those  por- 
tions of  the  main  ^ream,  or  hi  its  branches,  where  the  swift 
up-stream  reaches  deliver  their  load.s  to  less  rapidly  flowing; 


S  TREA  M  DE  VEL  OP  MEN  T 


225 


waters.  On  the  whole,  streams  carry  on  this  work  more 
rapidly,  and  lower  their  drainage  basin  to  baselevel  more 
quickly,  under  humid  than  under  rainless  skies.  That  is, 
increased  volume  favours  corrasion  and  hastens  the  coming 
of  the  final  stage  in  the  history  of  streams,  when  they  have 
reduced  their  basins  to  baselevel.  There  may  be  a  check  in 
this  increased  activity,  however,  due  to  greater  rain-fall, 
through  the  influence  of  more  luxuriant  vegetation. 

The  effects  of  a  long-continued  or  secular  decrease  in  pre- 
cipitation on  stream  development  and  erosion  are  illustrated 
on  a  small  scale  each  summer  in  the  eastern  portion  of  the 
United  States  and  Canada.  After  the  spring  rain,  as  is  well 
known,  the  monthly  precipitation  greatly  decreases,  the 
streams  become  less  and  less  in  volume;  many  of  the  weaker 
ones  disappear,  their  supplies  diminish,  their  waters  are 
evaporated,  or  absorbed  by  the  material  over  which  they 
flow.  When  one  follows  up  a  dry  stream-channel  in  late 
summer  he  comes  at  length  in  many  instances  to  where  it  is 
still  occupied  by  water.  The  trunk  of  the  drainage  system 
has  disappeared  and  the  branches  fail  to  unite.  The  streams 
have  been  betrunked  by  a  decrease  in  water  supply  and  an 
increase  in  loss  by  evaporation.  The  weakened  streams 
fail  to  abrade  their  channels  where  active  corrasion  was  in 
progress  a  few  months  previously,  and  all  the  debris  carried 
by  them  is  deposited  within  their  channels. 

On  account  of  decrease  in  load,  however,  many  streams 
are  enabled  to  corrade  their  channels  in  certain  tracts 
during  the  dry  season,  where  previously  overloading  led  to 
deposition. 

The  changes  experienced  by  streams  in  a  region  where 


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226 


RIVERS  OF  NORTH  AMERICA 


seasonal  changes  occur,  as  just  referred  to,  illustrate  the 
more  marked  results  of  secular  climatic  changes.  If  precipi- 
tation slowly  decreases  century  after  century,  even  the  larger 
rivers  may  fail  to  reach  the  sea,  but  annual  pulsations  in 
their  branches  and  even  in  the  main  channels  may  still  con- 
tinue. The  channels  of  the  main  streams  and  of  many  of 
their  tributaries  will  then  become  deeply  filled.  Some  of 
the  branches  will  fail  to  reach  the  main  stream-channel,  and 
the  drainage  tree,  as  it  appears  on  a  map,  is  betrunked. 

Many  streams  illustrating  an  advanced  stage  in  the  shrink- 
ing and  betrunking  of  once  vigorous  lines  of  drainage,  owing 
to  increased  dryness  of  climate,  occur  in  the  arid  portions 
of  the  United  States,  between  the  Rocky  Mountains  and 
the  Sierra  Nevada,  and  also  in  Texas  and  New  Mexico. 
An  examination  of  a  fairly  good  map  of  these  regions  will 
show  many  branches  of  previously  extensive  rivers,  which 
end  without  uniting  so  as  to  form  trunk  streams.  These 
lost  rivers  furnish  illustrations  of  the  dissection  of  streams 
by  a  secular  decrease  in  rain-fall  and  general  desiccation. 

In  other  instances,  as  in  the  case  of  Humboldt  River, 
Nevada,  a  trunk  stream  is  formed  ^rom  year  to  year  during 
the  rainy  season,  which  discharges  its  waters  on  to  a  broad 
desert  valley  where  they  spread  out  in  a  saline  lake  and 
are  evaporated.  During  the  next  succeeding  summer,  the 
trunk  stream  fails  to  reach  the  lake  into  which  it  previously 
discharged  and  shrinks  in  volume  throughout,  leaving 
isolated  ponds  in  the  deeper  portions  of  its  channel;  its 
branches  then  correspond  to  the  branches  of  a  fallen  tree 
from  which  the  trunk  has  been  removed. 

An  instructive  phase  of  the  drainage  of  an  arid  country  is 


S  TREA  M  DE  VEL  OP  MEN  T 


227 


intry  is 


furnished  when  a  stream  rising  in  more  humid  and  usually 
mountainous  regions,  flows  across  it,  and  has  sufficient  vol- 
ume to  sustain  the  losses  due  to  evaporation,  and  maintains 
its  course  to  the  sea.  Under  such  conditions  a  river  may 
continue  to  deepen  its  channel  through  an  arid  country  with- 
out developing  branches  in  its  middle  and  lower  courses. 
Such  a  stream,  returning  to  the  analogy  between  the  outline 
of  a  drainage  system  and  a  tree,  resembles  one  of  the  mon- 
archs  of  a  forest  having  a  tall  trunk  without  branches  until 
the  spreading  crown  of  foliage  is  reached.  An  example  of 
such  a  river,  with  an  immensely  long  trunk  from  which 
lateral  branches  have  fallen  away,  is  furnished  by  the  Colo- 
rado. The  trunk  of  the  Nile  measures  about  eleven  hundred 
miles  to  the  first  branch. 

The  character  of  the  trunk  of  a  river  flowing  through  an 
arid  country  will  depend  largely  on  the  altitude  of  the  land 
through  which  it  flows.  If  the  region  is  an  elevated  table- 
land, as  in  the  case  of  the  Colorado,  it  will  corrade  its  chan- 
nel down  to  baselevel  and  form  a  deep  canyon,  for  the  reason 
that  general  degradation  in  an  arid  climate  is  greatly  re- 
tarded. The  walls  of  the  canyon  will  be  precipitous  because 
the  stream  deepens  its  channel  more  rapidly  than  its  broad- 
ening cliffs  are  eroded.  If,  however,  the  arid  region  through 
which  the  river  flows  is  low,  aggrading  will  take  place. 

Between  the  two  extremes  just  cited,  many  intermediate 
phases  may  be  discovered.,  dependent  mainly  on  the  height 
of  the  land  above  the  sea.  When  the  topography  of  the 
region  through  which  flow  such  tall  drainage  trees,  as  we  may 
venture  to  call  them,  is  more  carefully  studied,  the  influence 
of  many  secondary  conditions  such  as  rock  texture,  charac- 


:|!. 


228 


RIVERS  OF  NORTH  AMERICA 


J  ! 


ter  of  the  infrequent  storms,  nature  of  the  vegetation,  etc., 
will  furnish  interesting  subjects  for  investigation. 

Variation  in  Temperature. — If  we  dissect  out,  as  it  were, 
the  effects  of  secular  variations  of  temperature  on  the  de- 
velopment of  streams,  from  other  climatic  conditions,  we 
find  that  an  increase  of  temperature  promotes  evaporation 
and  thus  decreases  the  volume  of  streams;  a  gradual  lower- 
ing of  the  mean  annual  temperature  for  a  series  of  decades, 
or  centuries,  will  favour  an  increase  in  the  volume  of  streams. 
But  secular  variations  in  temperature  mean  a  profound 
change  in  other  climatic  conditions,  and  start  a  wave  which 
is  felt  also  throughout  the  organic  kingdom  in  the  region 
affected.  The  resultant  changes  in  fauna  and  flora  react  on 
the  inorganic  kingdom,  and  notably  on  the  volume,  varia- 
tions, and  development  of  streams.  An  increase  in  mean 
annual  temperature  in  many  regions  is  followed  by  a  greater 
luxuriance  of  the  flora;  while  over  other  extensive  areas,  as, 
for  example,  in  arid  and  desert  countries,  it  means  a  decrease 
in  the  previously  scanty  vegetation.  A  reverse  wave  of  solar 
energy  means  in  general  a  decrease  in  the  luxuriance  in  the 
flora  of  previously  warm  and  humid  regions  and  an  increase 
in  the  vegetation  of  previously  hot  and  arid  regions ;  while 
cold,  humid  regions  will  become  still  more  arctic  and 
arboreal  vegetation  decline.  Some  of  the  influences  of 
plant-life  on  the  behaviour  and  growth  of  streams  have 
already  been  noted  and  others  will  be  considered. 

The  variation  in  temperature  which  most  directly  affects 
the  streams  is  a  change  from  temperate  to  arctic  conditions. 
A  lowering  of  the  mean  annual  temperature  in  regions 
which  previous  to  the  change  had  a  temperate  or  sub-arctic 


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STREAM  DEVEI.OPMEiVT 


229 


climate,  means  an  increase  in  the  length  of  the  winters  and 
a  shortening  of  the  summers.  A  progressive  and  cumula- 
tive change  in  this  dir(  tion  would  be  accompanied  ordinarily 
by  an  increase  in  the  winter's  snow-fall  and  the  preservation 
of  the  snow  until  late  in  the  spring  or  even  far  into  the 
summer,  and  when  the  accumulated  snows  finally  yielded  to 
the  increasing  temperature  of  the  warm  season,  floods  would 
result.  While  the  ground  was  frozen  and  snow-covered^ 
erosion  would  be  nil,  the  streams  clear  and  ice-bound,  and 
all  the  varied  processes  of  corrasion  and  deposition  greatly 
retarded.  With  the  breaking  of  the  river  ice  when  warm 
rains  melted  the  snow,  the  increased  volume  of  the  streams, 
a  generous  supply  of  debris  loosened  by  the  winter's  frost, 
and  yet  other  conditions  would  favour  stream  work.  In 
many  ways,  therefore,  a  moderate  or  at  least  not  excessive 
lowering  of  the  mean  annual  temperature,  especially  in  pre- 
viously temperate  and  sub-arctic  regions,  would  be  accom- 
panied by  increased  activity  in  stream  development.  A 
more  excessive  refrigeration  would  lead  to  th.  accumulation 
of  perennial  snow  and  the  formation  of  glaciers. 

Fluctuations  of  Strcauis. — The  amount  of  water  flowing 
through  most  stream  channels  is  subject  to  many  fluctua- 
tions. The  most  constant  streams  are  such  as  are  fed  by 
large  springs  or  by  the  overflow  of  broad  lakes,  for  the 
reason  that,  in  instances  of  this  nature,  the  water  comes 
from  reservoirs  of  such  size  as  not  to  be  materially  influenced 
by  sudden  rains  or  by  ordinary  droughts.  The  subterranean 
reservoirs  referred  to  as  supplying  springs  are  not  usually 
definite  water-bodies  like  the  lakes  in  caverns,  but  masses  of 
porous  soil  and  rock  which  became  saturated  by  percolation. 


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RIVERS  OF  NORTH  AMERICA 


It  is  to  be  noted,  however,  thrt  streams  flowing  from 
large  lakes  do  experience  some  variation  in  volume,  as  when 
a  strong  wind  blowing  over  a  lake  causes  its  waters  to  rise 
on  the  side  towards  which  the  wind  blows ;  when  this  side 
is  the  one  from  which  the  vvater  escajjes,  a  sudden  rise  is 
experienced  in  the  draining  stream.  Pv  reverse  direction  of 
the  wind  would  cause  a  diminution  in  the  volume  of  the 
outflowing  water.  There  are  also  other  atmospheric 
changes,  such  as  those  producing  sudden  variations  in  the 
level  of  laf-^e  water,  known  as  seiches,  which  at  times  have  a 
temporary  influence  on  the  volume  of  streams  flowing  from 
lakes. 

Th.:  principal  influences  which  cause  streams  to  vary  in 
volume,  although  diverse  and  frequently  complex,  may  be 
provisionally  classified  in  three  groups,  namely,  seasonal 
changes  of  climate,  weather  changes,  and  the  breaking  of 
dams.  There  are,  besides,  and  perhaps  most  important 
of  all,  secular  climatic  changes,  like  those  that  accompanied 
the  Glacial  epoch,  to  which  the  greatest  variations  in  the 
histories  of  streams  are  due,  but  these  are  beyond  our 
immediate  dreams. 

Of  the  changes  in  climatic  conditions  accompanying  the 
orderly  march  of  the  season,  those  producing  alternate  wet 
and  dry  periods  have  the  most  direct  and  conspicuous  influ- 
ence on  the  volume  of  the  streams  This,  however,  is  a 
matter  of  every-day  knowledge,  and  need  claim  but  little 
space  at  this  time. 

Throughout  nearly  the  whole  of  North  America,  the  rainy 
season  is  also  the  cold  season ;  the  ground  is  frequently 
frozen,    thus   checking   percolation,  and  the  country  over 


STREAM  DEVELOPMENT 


231 


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large  areas  is  snow-covered.  Wher.  a  thaw  occurs,  the 
water  formed  by  the  melting  of  the  snow,  increased  perhaps 
by  a  copious  rain-fall,  flows  quickly  over  the  frozen  soil  and 
causes  floods  in  the  valleys. 

At  times  when  the  ground  is  not  frozen,  a  portion  of  the 
surface  water  flows  directly  to  the  stream,  while  another, 
and  frequently  the  larger  portion,  sinks  below  the  surface 
and  percolates  slowly  away.  Under  these  conditions,  there 
are  commonly  two  well-marked  periods  of  rise  in  the  adja- 
cent streams  ;  the  first  and  quickest  rise  following  the 
inrush  of  surfpxe  run-off,  and  che  second  usually  involving 
a  greater  volume  of  water,  caused  by  the  sub-surface  water 
percolating  out  from  the  sides  of  valleys  and  flowing  from 
springs.  There  is  marked  difference  betVv'een  these  two 
floods  referred  to  in  various  regions,  dependent  on  the 
permeability  of  the  soil  and  of  the  rock  immediately  be- 
neath. When  the  material  composing  the  hills  and  valley 
sides  is  deep  and  porous,  the  first  rise  is  small,  unless  the 
rain-fall  is  remarkably  heavy,  and  quickly  subsides,  while 
the  slower  rise  which  follows  is  of  longer  duration  and  of 
much  greater  volume.  In  regions  with  an  impervious 
covering,  such  as  clay,  the  first  rise  of  the  stream,  after  a 
heavy  rain-fall,  is  marked  both  by  its  suddenness  and  by  the 
large  volume  of  the  run-off.  The  second  rise  may  then  be 
slight,  or  even  not  recogniiiable. 

Each  of  the  two  conditions  of  soil  referred  to  is  ac- 
companied by  characteristic  topographic  forms.  In  regions 
covered  by  impervious  soil,  as  in  the  case  of  very  large 
areas  in  the  South  Atlantic  States  where  residual  clays  pre- 
dominate,   the    land    is    everywhere    trenched    by    stream 


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232 


RIVERS  OF  NORTH  AMERICA 


channels,  v^hich  ensure  a  quick  escape  for  surface  water. 
On  the  other  hand,  where  deep,  porous  soils  occur,  as  in 
much  of  the  formerly  glaciated  region  of  the  United  States, 
the  surface  waters  are  quickly  absorbed,  and  percolate 
slowly  away,  leaving  broad  areas  between  the  larje  stream- 
cut  valleys  without  rill  marks. 

Floods  in  streams  are  caused  by  seasonal  changes  in  rain- 
fall and  also  by  the  melting  of  the  snow  on  high  mountains. 
In  early  summer,  for  example,  when  the  snow  is  melting  on 
the  Rocky  Mountains,  the  Missouri  and  the  lower  Mississippi 
undergo  what  is  known  as  the  June  rise.  Rain-fall  floods 
follow  excessive  rains  and  may  occur  in  a  part  or  the  whole 
of  a  river  system.  When  the  swollen  tributaries  of  a  trunk 
stream  deliver  their  waters  to  the  main  channel  at  the  same 
time,  great  floods  occur  below  where  the  branches  unite. 
This  class  of  floods  can  be  readily  predicted  when  a  suffi- 
cient number  of  weather  records  and  of  gauge  readings  in 
the  principal  tributaries  are  available.  Valuable  work  in 
this  connection  is  being  done  by  the  United  States  Weather 
Bureau.  The  floods  of  the  Ohio  are  of  this  character,  but 
the  highest  rises  are  augmented  by  the  melting  of  winter 
snow.  The  highest  waters  occur  usually  in  February,  when 
a  height  of  over  fifty  feet  above  the  summer  stage  frequently 
occurs. 

The  waters  of  streams  are  in  many  cases  held  in  check  b/ 
glacial  dams,  ice-Ljorges,  accumulations  of  drift-wood,  land- 
slides, avalanches,  etc.,  until  the  breaking  of  the  obstruction 
permits  of  the  emptying  of  the  reservoirs  above  them,  and 
floods  sometimes  accompanied  by  great  loss  of  life  and  de- 
struction of  property  occur  in  the  valleys  below.     Floods  of 


STREAM  DEVELOPMENT 


235 


this  character  can  seldom  be  predicted  for  any  consider- 
able time  in  advance,  and  to  this  fact  is  due  much  of  the 
destruction  wrought  by  them. 

Inundations  of  portions  of  valleys  and  flood-plains,  not 
usually  :n  danger  during  annual  high-water  stages  of  the 
streams  flowing  through  them,  occur  when  the  necks  of 
land  left  by  the  migrc^Lion  of  streams  are  cut  through  and  a 
new  channel  is  rendered  available.  The  aggrading  of  stream 
channels  on  alluvial  cones  and  deltas  leads  to  similar  results. 
The  clogging  of  stream  channels,  owing  to  abundant  deposi- 
tion, also  causes  them  to  overflow  their  banks,  especially 
during  the  annual  high-water  stages. 

River  floods  are  now  being  systematically  studied  in  the 
regions  occupied  by  the  more  civilised  nations,  and  most 
valuable  results  may  be  expected  from  these  scientific  in- 
vestigations, particularly  in  the  direction  of  predicting  when 
floods  are  to  be  expected,  thus  allowing  opportunities 
to  counteract  their  destructive  effects,  and  to  utilise  the 
inundation  of  farm  lands  so  as  to  enrich  them  by  the 
silt  thrown  down.  Much  of  the  richest  land  in  the  world 
is  situated  on  the  flood-plains  of  great  rivers.  These 
plains  owe  their  origin,  as  has  been  already  ^^xplained, 
to  the  deposits  made  during  high-water  stages  of  the^ 
streams.  In  general,  it  seems  better  for  man  to  adapt 
his  industries  to  these  natural  conditions  and  endeavour  to 
utilise  the  floods  to  enrich  his  land,  rather  than  to  build 
levees  for  the  purpose  of  preventing  inundations.  This 
branch  of  river  study  falls  properly  in  the  field  of  the  en- 
gineer, and  is  too  technical  to  be  treated  at  length  at  this 
time. 


i\ 


234 


RIVERS  OF  NORTH  AMERICA 


»•■■■ 


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SOME  OF  THE   INFLUENCES  OF   GLACIERS  ON   STREAM 

DEVELOPMENT 

The  origin,  growth,  and  retreat  of  glaciers  in  a  region  pre- 
viously  occupied  by  streams  have  a  varied  influence  on  their 
lives.  In  the  case  of  alpine  glaciers  originating  about 
isolated  peaks  or  on  the  summits  of  mountain  ranges,  the 
previously  excavated  stream -valleys  become  avenues  of  ice- 
drainage,  and  in  the  more  elevated  portions  of  such  valleys 
all  stream  work  is  stopped.  Subglacial  streams  originate 
which  are  heavily  charged  with  silt,  and,  judging  from 
existing  examples,  would  be  in  a  condition  to  abrade  the 
rocks  over  which  they  flowed.  The  streams  below  the  ends 
of  the  glaciers,  and  supplied  by  their  melting,  would  be 
swollen  in  summer  and  greatly  diminished  in  volume  in 
winter.  Their  efficiency  as  transporting  and  corrading 
agencies  would  thus  be  increased  beyond  the  power  they 
would  have  if  the  water  supplied  to  them  was  delivered  with 
more  uniformity.  But  the  chief  effect  of  such  a  change  as 
has  just  been  postulated,  as  is  shown  by  the  study  of  exist- 
ing glaciers,  would,  in  general,  be  the  overloading  of  the 
streams  below  ^.here  the  drainage  changed  from  a  solid  to  a 
liquid  form.  Alpine  glaciers  normally  deliver  to  their  drain- 
age streams  more  debris  than  the  latter  are  able  to  transport, 
and  aggrading  begins  at  their  very  sources  unless  the  stream 
channels  are  remarkably  precipitous.  Many  valleys  at  the 
present  day,  which  are  occupied  by  glaciers  of  the  alpine 
type  in  their  upper  courses,  are  deeply  filled  with  debris  all 
the  way  from  the  lower  extremities  of  the  glaciers  to  the 
lake  or  sea  into  which  they  discharge.     Broad  flood-plains, 


STREAM  DEVELOPMENT 


235 


bifurcating  streams,  and  valley  sides  without  terraces  are 
some  of  the  more  striking  features  of  valleys  which  have 
experienced  such  changes. 

The  modifications  in  the  behaviour  of  streams  flowing 
from  an  isolated  peak  or  mountain  range  on  which  glaciers 
have  been  developed  at  the  heads  of  previously  excavated 
stream-valleys,  are  similar  to  the  changes  in  the  lives  of 
streams  draining  a  region  where  a  continental  glacier 
originates  and  expands. 

As  a  continental  glacier  advances  and  occupies  a  pre- 
viously well-drained  region,  the  streams  are  obliterated  over 
most  of  the  area  that  becomes  occupied  by  the  ice.  Near 
the  outer  border  of  the  ice-sheet,  however,  and  perhaps  for 
scores  of  miles  back  from  the  margin  and  beneath  the  ice, 
preglacial  streams  may  continue  to  flow  or  new  subglacial 
streams  originate.  The  gradients  of  these  streams  will  not 
be  high,  and,  judging  from  the  subglacial  stream  recorders 
in  regions  occupied  by  Pleistocene  glaciers,  will  be  over- 
loaded and  consequently  forced  to  drop  debris  and  raise  the 
beds  of  their  channels.  When  the  streams  leave  the  ice  and 
expand  and  bifurcate,  as  usually  happens,  their  velocities 
will  be  still  further  decreased  and  still  more  of  their  loads 
deposited.  Flood-plains  and  alluvial  cones  me'ging  into 
sand  and  gravel  plains  will  be  the  more  striking  results  of 
such  conditions.  When  the  streams  leave  their  tunnels  in 
the  ice,  aggrading  may  be  so  active  that  their  beds  will  be 
raised  and  a  check  imposed  upon  the  flow  of  their  waters 
through  the  tunnels  from  which  they  bring  their  loads  of 
debris,  thus  necessitating  the  raising  of  the  bottoms  of  the 
tunnels  and  their  enlargement  above  by  melting  the  ice. 


n  *'i 


i 

i 
1 

1 

236 


RIVERS  OF  NORTH  AMERICA 


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This  process,  by  which  ridges  of  grave)  and  sand,  or  osars, 
are  formed  beneath  ice-sheets  both  of  the  continental  and 
piedmont  types,  has  been  considered,  to  some  extent,  in  a 
former  chapter,  and  at  greater  length  in  a  previously  pub- 
lished volume  by  the  present  writer.' 

When  a  continental  glacier  wastes  away  and  its  outer 
border  retreats,  the  zone  of  marginal  deposition  recedes 
also,  and  will  be  followed  in  turn  by  conditions  which  favour 
corrasion,  and  the  previously  formed  flood-plains,  etc.,  will 
have  channels  and  valleys  cut  through  them. 

The  considerations  just  presented  are  necessarily  of  the 
nature  of  an  outline  sketch,  but  I  trust  furnish  sufficient 
suggestions  to  enable  the  reader  to  fill  in  the  details  in  the 
case  of  any  special  field  that  may  come  under  his  notice. 

SOME   OF  THE    INFLUENCES    OF  VEGETATION    ON    STREAM 

DEVELOPMENT 

In  a  region  bare  of  all  vegetation  and  where  the  surface  is 
inclined,  the  rain-water  gathers  quickly  into  rills  and  rivu- 
lets which  at  once  begin  to  excavate  channels.  When  the 
ground  is  grass-covered,  it  is  protected  from  the  impact  of 
the  falling  drops,  and  the  plant  roots  bind  the  soil  together 
so  as  to  greatly  retard  its  removal.  If  shrubs  and  trees  rise 
above  a  grass-covered  surface,  still  greater  protection  is 
afforded.  The  tendency  of  vegetation  is  thus  to  shield  the 
land  and  allow  surface  water  to  be  absorbed  by  the  soil  and 
percolate  quietly  away,  thus  robbing  it  of  the  power  to  cor- 

'  I.  C.  Russell,  Glaciers  of  North  America,  pp.  28,  29,  123-125.  Ginn  & 
Co.,  1897.  The  name  osar  was  adopted  in  this  volume  in  place  of  eskar^  kamt, 
or  osar,  in  conformity  with  American  usage.  i." 


STREAM  DEVELOPMENT 


237 


rade.  This  is  illustrated  especially  in  humid  regions  where 
ploughed  fields  are  adjacent  to  meadows,  pastures,  and 
woodlands.  Ploughed  fields,  if  neglected  for  a  few  years, 
will  become  channelled  by  stream  courses,  and  immature 
drainage  systems  developed  where  the  slopes  are  favourable, 
while  adjacent  fields  covered  with  grass  and  other  pla.its 
are  not  visibly  affected.  The  abandoned  fields  of  Virginia 
and  the  more  southern  Atlantic  States,  cut  by  thousands  of 
gullies,  furnish  sad  testimony  of  the  disasters  which  follow 
the  breaking  of  the  soil  in  regions  where  the  waters  are 
retained  at  the  surface  and  gather  into  rills  instead  of  per- 
colating slowly  away.  If  the  soil  and  subsoil  are  deep  and 
porous,  however,  the  fields  may  not  be  seriously  r'fected 
by  the  breaking  of  the  soil. 

While  vegetation  retards  mechanical  corrasion,  it  favours 
the  chemical  action  of  percolating  waters  by  supplying  them 
with  organic  acids,  as  has  already  been  explained,  and 
hastens  chemical  corrasion. 

Vegetation  has  an  effect  on  streams  also,  from  the  fact 
that  it  promotes  evaporation  and  retards  the  gathering  of 
surface  waters  in  channels.  Evaporation  is  favoured  by  the 
retention  of  the  water,  and  yet  still  more  efficiently  through 
the  vital  functions  of  the  plants  themselves.  Water  is 
taken  in  by  the  rootlets  and  escapes  from  the  leaves.  The 
amount  of  water  returned  to  the  atmosphere  in  this  manner 
from  areas  clothed  with  luxuriant  vegetation  is  surprisingly 
great.  It  has  been  stated  that  to  grow  a  ton  of  hay  the 
grass  plants  must  drink  in  two  to  three  hundred  tons  of  water.' 

'J.  W,  Powell,  National  Gecgmphic  Monographs,  vol.  i.,  p.  69,  published 
by  the  American  Book  Co.,  New  York,  1895. 


"i 

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1 

1ili 


J  !i 


238 


mVEJiiS  OF  NORTH  AMERICA 


*  t  ? 


The  rjm-off,  as  the  escape  of  rain-water  by  surface  streams 
is  termed,  in  plant-covered  regions  is  retarded  and  the 
volume  of  the  draining  streams  rendered  more  uniform 
than  it  would  be  should  the  vegetation  be  removed.  The 
work  of  streams  is  thus  checked  in  two  conspicuous  ways  by 
vegetation :  first,  by  the  decrease  in  surface  waters  due  to 
evaporation,  and  second,  by  diminishing  floods. 

When  a  mat  of  vegetation  mantles  the  ground,  as  espe- 
cially when  broad  areas  are  deeply  moss-covered,  as  in  much 
of  Alaska  and  about  the  shores  of  Puget  Sound,  the  surface 
waters  are  filterea  of  practically  all  material  in  suspension 
and  gather  into  rills  which  are  clear,  although  frequently 
amber-coloured  on  account  of  organic  matter  in  solution, 
and  hence  have  but  slight  powers  to  abrade  the  rocks  over 
which  they  flow. 

The  influence  of  the  roots  of  trees,  and  particularly  of 
willows,  alders,  cottonwoods,  etc.,  which  flourish  along  the 
margins  of  streams,  on  erosion  of  the  banks  is  well  known. 
Such  vegetation  assists  particularly  in  retaining  the  sediment 
of  streams  when  they  expand  and  flood  the  adjacent  land, 
thus  assisting  materially  in  the  formation  of  natural  levees. 

The  evil  efl"ects  of  removing  the  forests  from  a  mountain- 
ous region  are  seen  especially  in  the  quicker  and  greater  run- 
off of  the  waters  falling  on  it.  This  is  accompanied  by 
more  rapid  erosion  in  the  upper  courses  of  the  stream  and 
aggrading  in  the  valleys  lower  down.  Flood  plains  are  thus 
raised  where  the  gradients  of  the  streams  decrease,  and  pre- 
viously fruitful  areas  along  the  borders  of  the  rivers  may 
be  converted  into  barren  tracts  of  gravel  and  sand,  which 
remain  for  a  long  time  unavailable  for  agriculture. 


as  espe- 
in  much 
i  surface 
jpension 
iquently 
lolution, 
cks  over 


Plate  XII. 


1 


Fig.  a.     Beaver  Dam,  Wyoming. 
(Photograph  by  W.  H.  Jackson.) 


larly  of 
ong  the 
known, 
ediment 
nt  land, 
1  levees. 
Duntain- 
Lter  run- 
nied  by 
:am  and 
are  thus 
and  pre- 
ers  may 
i,  which 


Fig.  B.     Dam  of  Drift-Wo'jd,  Teanaway  River,  Washington. 


;3 

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STREAM  DEVELOPMENT 


239 


The  influence  of  vegetation  on  stream  development,  as 
shown  even  by  this  hasty  and  incomplete  review,  is  highly 
complex.  In  fact  the  influences  of  vegetation  counteract 
each  other,  as  when  dense  plant  growth  and  a  layer  of  de- 
caying leaves  and  branches  retard  mechanical  corrasion,  but 
by  supplying  organic  acids  to  percolating  waters,  promote 
chemical  corrasion. 

In  forest-covered  regions,  especially  where  the  ground  is 
encumbered  with  undergrowths,  or  where  mosses  and  li- 
chens form  a  thick  mantle,  the  run-off  is  retarded  and  stream 
development  delayed.  When  the  land  is  grass-covered,  as 
in  prairie  regions,  where  in  general  the  rain-fall  is  moderate, 
erosion  is  checked.  In  many  instances,  no  mechanical  erosion 
occurs  in  prairie  regions,  because  all  the  water  that  falls  is 
absorbed  by  the  deep  porous  soil  and  evaporated  largely 
through  the  agency  of  plants,  or  percolates  slowly  away. 

In  regions  imperfectly  clothed  with  desert  shrubs,  and 
where  the  bunch-grass  common  to  such  localities  does  not 
form  a  sod,  as  in  the  thousands  of  sage-brush  valleys  of  the 
arid  regions  of  North  America,  the  scanty  rain-fall  ib  apt  to 
come  in  the  form  of  violent  storms  and  copious  downpours. 
The  water  being  unchecked  by  vegetation  gathers  quickly 
into  streams  wherever  the  surface  slope  is  favourable,  and 
rushes  along  bearing  with  it  heavy  loads  of  debris.  Be- 
tween the  infrequent  storms  the  stream  channels  become 
clogged  with  debris  swept  in  by  the  wind  or  slowly  creeping 
down  their  sides  under  the  influence  of  changes  of  tempera- 
ture and  other  agencies.  When  the  next  "  cloud-burst  " 
comes,  perhaps  after  the  lapse  of  scores  of  years,  the  ac- 
cumulated debris  is  again  swept  onward. 


J1\^:« 


w^mm 


240 


RIVEKS  OF  A'OA'T//  AAfEKICA 


111 


11 


The  conditions  most  favourable  for  general  mechanical 
degradation  and  stream  corrasion  and  deposition,  so  far  as 
vegetation  is  concerned,  are  when  hind  is  bare  or  but  scan- 
tily clothed  with  plants  of  any  kind.  If,  in  addition,  the 
annual  rain-fall  is  spasmodic,  insteail  of  the  same  amount 
of  waver  falling  on  a  given  area  being  evenly  distributed 
throughout  the  year,  exceedingly  favourable  conditions  for 
the  development  of  streams  result,  providing  the  requisite 
slopes  are  present.  - 

These  conditions  are  more  nearly  fulfilled  in  the  arid, 
south-west  portion  of  the  United  States  than  in  any  other 
part  Oi  North  America,  and  it  \y  there  that  the  labours  of 
Newberry,  Powell.  Gilbert,  Dutton,  and  others  have  been 
fruitful  in  such  great  results  in  the  way  of  interpreting  the 
origin  of  topog.-aphic  forms. 

Drift- U\>0(/. — In  considering  the  mechanical  work  of 
streams  from  either  an  engineering  or  a  purely  geographical 
point  of  view,  account  needs  to  be  taken  of  the  influence  of 
the  drift-wood  carried  by  them.  In  many  instances  the 
amount  of  floating  timber  and  of  trees  lodged  against  the 
bottom  and  sides  of  a  stream  is  so  great  as  seriously  to  im- 
pede navigation,  and  not  infrequently  to  render  it  impos- 
sible. Drift-wood,  al«:,o,  in  one  way  or  another,  botn  assists 
and  retards  erosion,  and  by  diverting  streams  from  their 
channels  leads  to  important  geographical  changes. 

As  is  well  known,  streams  flowing  through  forested  regions 
receive  great  numbers  of  trees  which  fall  into  them  princi- 
pally on  account  of  the  undercutting  and  consequent  cav- 
ing of  their  banks.  Fallen  trees  are  swept  into  streams 
•during  floods,  and  when  ice-gorges  occur  gre.it  destruction 


kp 


STKEAM  DE  VKL  OP  MEN  T 


241 


of  timber  sometimes  results.  In  ascending  such  rivers  as 
the  Mississippi  or  the  Yukon,  one  frequently  sees  whole 
trees,  with  branches  and  roots  attached,  floating  with  the 
current.  In  manv  such  instances,  the  branches  or  roots 
drag  on  the  bottom  and  disturb  the  mud  or  sand,  and  thus 
aid  in  the  transportation  of  rock  debris.  At  times  the 
drifting  trees  become  water-logged,  and  sink  to  the  bottom 
in  such  a  manner  as  to  become  anchored  at  one  end,  and 
being  swayed  by  the  current  again  assist  corrasion  by  causing 
eddies  in  the  water,  as  well  as  by  the  direct  agitation  of  the 
material  in  which  their  roots  or  branches  are  embedded. 

Drift  wood  carried  by  swift  streams,  and  especially  during 
floods,  tends  to  leave  the  belt  of  most  rapid  flow  and  ac- 
cumulate along  the  banks,  thus  greatly  increasing  the 
chances  of  its  becoming  lodged.  Trees  that  have  fallen 
from  the  bank  of  a  stream,  but  are  yet  held  by  their  roots, 
cause  obstructions  and  retard  the  progress  of  floating  tim- 
ber. In  these  and  still  other  ways  the  trees  which  fa'I  into 
streams  have  a  conservative  influence  and  tend  to  retard 
lateral  corrasion. 

The  conservative  tendencies  of  drift-wood  are  also  seen  on 
lake  and  ocean  shores  where  it  is  thrown  on  the  beach,  ant 
becoming  partially  or  wholly  buried  in  sand  and  mud,  serves 
to  bind  the  shore  accumulations  together  and  counteract 
the  force  of  the  waves.  The  protection  afforded  by  stranded 
drift-wood  is  especially  well  illustrated  on  the  borders  of  the 
Laurentian  lakes,  and  on  the  beaches  of  Puget  Sound. 
At  each  of  these  localities  one  may  frequently  walk  for 
several  miles  h\  stepping  from  one  strandc  J  log  ^o  another. 

In  each  of  the  instances  referred  to,  however,  the  natural 

16 


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242 


RIVERS  OF  NORTH  AMERICA 


accumulation  of  drift-wood  is  augmented  by  the  waste  from 
saw-mi.ls.  v   ■ 

The  process  by  which  the  beaches  of  lakes,  sounds,  etc., 
are  protected  by  stranded  timber  is  in  action  also  along 
river  banks,  but  the  ability  of  the  currents  in  removing  such 
obstructions  is  then  usually  more  pronounced. 

The  conservative  influence  of  drift-wood  on  n'ver  banks  is 
well  shown  at  numerous  localities  along  the  Yukon,  more 
especially  whore  the  river  divides  so  as  to  enclose  islands. 
Where  small  branches  of  the  river,  or  bayous,  leave  the 
main  stream,  as  has  already  been  mentioned,  there  are  fre- 
quently large  accumulations  of  stranded  drift-wood.  These 
"  wood-yards  "  are  in  numerous  instances  several  acres  in 
area,  and  from  fifteen  to  twenty  or  more  feet  deep.  In 
many  localities,  as  has  been  observed  by  the  writer,  the  en- 
trances to  bayous  have  been  completely  closed  by  these  ac- 
cumulations, the  interstices  between  the  logs  and  broken 
branches  being  clogged  with  mud  and  sand,  so  that  practi- 
cally no  water  enters  the  side  channels  except  when  tho  river 
is  in  flood.  When  the  up-stream  ends  of  the  bayous  are 
dammed  in  this  manner,  their  down-stream  extremities  be- 
come silted  up  and  they  arc  transformed  into  lakelets.  The 
accumulation  of  drift-wood  thus  tends  to  confine  the  river 
to  its  main  channel  and  retard  lateral  corrasion.      < 

In  mountainous  regions  where  forests  thrive,  the  streams 
arc  sometimes  completely  dammed  by  accumulations  of 
drift-wood  and  forced  to  excavate  new  channels.  Thes.c 
dams  are  usually  started  by  the  falling  of  a  large  tree  across 
a  stream,  which  holds  drift-wood  brought  from  above.  In 
this  way  a  skeleton  dam,  as  it  were,  is  formed,  but  the 


f 


S  TREA  M  DE  I  'ELOPMEN  T 


243 


openings  are  apt  to  become  clogged  with  smaller  branches 
and  leaves,  and  then  more  or  less  completely  filled  with  sand 
and  mud.  Several  instances  of  this  nature  have  come  under 
the  writer's  notice  in  the  Cascade  Mountains.  The  middle 
fork  of  Teanav/ay  River,  Washington,  for  example,  in  several 
localities  has  been  completely  turned  from  a  form.er  course 
by  dams  of  the  nature  just  described,  and  caused  to  ex- 
cavate a  new  channel.  A  view  of  one  of  these  natural 
log-dams  is  presented  on  Plate  XII.  In  this  instance,  a 
former  channel  has  been  filled  with  drift-wood  to  a  depth 
of  about  twenty  feet  for  a  distance  of  some  three  hundred 
yards  and  the  stream  completely  diverted.  At  the  upper 
end  of  the  obstruction,  sand  and  gravel  have  been  deposited 
against  the  drift-wood  to  the  depth  of  several  feet,  and  now 
form  the  bank  of  the  stream  where  it  leaves  its  former  course. 
The  most  remarkable  example  of  the  influence  of  accumu- 
lations of  drift-wood  on  the  behaviour  of  streams  yet 
reported  in  North  America  is  probably  furnished  by  the 
timber  rafts,  as  they  are  termed,  in  Red  River,  Louisiana. 
These  rafts  are  several  square  miles  in  area,  and  have  been 
in  existence  so  long  that  soil  has  formed  on  them  in  which 
trees  have  taken  root  and  flourished  so  as  to  produce  a 
floating  forest.  It  is  stated  by  Humphreys  and  Abbot  ' 
that  these  rafts  contain  an  immense  accumulation  of  tree- 
trunks,  some  floating,  and  others  so  water-logged  as  to  sink 
and  thus  still  more  effectually  to  block  the  channel.  From 
the  rotting  of  the  logs  at  the  lower  ends  of  the  rafts  and 
fresh  accumulations  at  their  upper  "nds,  they  are  gradually 
migrating  up  stream.     These  obstructions  tend  to  pond  the 

'  Ripot  t  OH  the  Mississippi  River,  1861,  p.  37. 


1 

m/ 


244 


RIVERS  OF  NORTH  AMERICA 


w. 


river  and  cause  it  to  form  lake-like  expansions,  which  some- 
times discharge  through  new  outlets.  The  influence  of  the 
rafts  on  Red  River  is  much  the  same  as  in  the  smaller  in- 
stance noted  above  in  the  case  of  Teanaway  River,  and 
illustrates  the  disturbances  which  the  streams  of  forested 
regions  frequently  experience. 

SUPERIMPOSED    STREAMS 

The  reader  is  already  aware  that  a  consequent  stream 
follows  a  predetermined  course  inherited  from  pre-existing 
surface  conditions,  and  is  at  first  not  influenced  by  the 
structure  of  the  rocks  beneath  the  surface.  It  also  happens 
that  a  stream  may  have  its  course  determined  by  rocks  that 
existed  above  the  surface  now  exposed,  but  which  have  been 
removed,  and  may  simulate  a  consequent  stream. 

If  we  imagine  an  ice-sheet  to  have  covered  a  region  of 
mild  relief,  and  that  streams  flowing  over  the  surface  of 
the  ice  were  slowly  lowered  upon  the  land  beneath  as  the 
ice  melted,  they  might  entrench  themselves  in  the  rocks  so 
as  to  hold  their  former  courses  after  the  ice  entirely  disap- 
peared. The  streams  might  have  their  direction  of  flow 
determined  to  some  extent  by  the  surface  features  of  the 
land  uncovered  by  the  melting  of  the  ice,  but  they  would 
be  uninfluenced  by  the  structure  of  the  rocks  underlying 
the  exposed  surface. 

Not  only  glaciers,  however,  but  geological  formations  arc 
removed  from  the  superficial  portions  of  large  areas. 

To  present  another  hypothetical  case:  imagine  a  region 
of  foldgd  and  faulted  rocks  to  have  been  worn  down  to  a 
peneplain    and    then   submerged    beneath    the   ocean   and 


STREAM  DEVELOPMENT 


245 


covered  with  a  horizontal  sheet  of  sediment.  Should  such 
an  area  be  again  upraised,  consequent  streams  would  be 
developed  on  its  surface  and  begin  an  orderly  sequence 
of  changes  as  they  advanced  with  the  task  of  again  reduc- 
ing the  land  to  baselevel.  If  the  buried  peneplain  was  above 
the  new  baselevel,  the  master  streams  would  cut  through 
the  covering,  presumably  of  soft  rocks,  resting  on  the  old 
peneplain  and  sink  their  channels  into  it.  Should  the 
covering  of  soft  rock  be  now  removed,  the  streams  would 
flow  across  the  uncovered  region  in  courses  determined  by 
the  surface  slope  of  the  cover  on  which  they  originated,  and 
wHhout  reference  to  the  surface  features  of  the  exposed 
plain  or  to  the  structure  of  the  rocks  beneath  it.  A  drain- 
age system  inherited  in  this  manner  by  one  geological 
terrane  from  another  is  said  to  be  superimposed. 

Such  a  history  as  has  just  been  outlined  furnishes  an  ex- 
planation of  the  geography  of  certain  regions  which  cannot 
be  satisfactorily  accounted  for  in  any  other  way. 

In  east-central  New  Jersey  we  have  a  coastal  plain  some 
twenty-five  to  thirty  miles  broad.  Rising  from  this  plain 
there  are  long,  narrow  ridges  of  hard  igneous  rock,  such  as 
the  double  ridge  known  as  the  Watchung  Mountains,  the 
Palisades  of  the  Hudson,  and  several  other  smaller  hills  and 
ridges  of  the  same  general  character.  These  ridges  have 
remarkably  even  crest-lines,  but  are  notched  in  places  by 
water-gaps  and  wind-gaps.  As  has  been  adnirably  worked 
out  by  Davis  and  Wood,'  the  tops  of  these  ridges  are  por- 

'  W.  M.  Davis  and  J.  Walter  Wood,  "  The  (ieographical  Development  of 
Northern  New  Jersey,"  in  Boston  Society  of  Natural  History,  Proiccdings^ 
vol.  xxiv.,  pp.  365-423,  1889. 


4 

i 


246 


RIVERS  OF  NORTH  AMERICA 


•A 


\% 


i  I 


tions  of  an  ancient  peneplain  which  has  been  upraised  some- 
what irregularly,  and  eroded  so  as  to  leave  the  edges  of  the 
sheets  of  hard  rock  which  traverse  it  in  bold  relief.  Previous 
to  the  upraising  of  the  peneplain  it  was  depressed  beneath 
the  sea  and  soft  sedimentary  beds  spread  over  it,  which 
reached  nearly  if  not  completely  across  the  central  part 
of  the  State.  The  peneplain  with  its  cover  of  soft  rock 
was  then  raised  and  a  system  of  consequent  streams  came 
into  existence.  The  streams  cut  through  in  the  soft  sur- 
face beds,  were  lowered  to  the  concealed  peneplain  be- 
neath, and  continued  to  deepen  their  channels.  The 
streams  flowed  across  the  edges  of  the  hard  beds  and  carved 
notches  in  them.  When  the  soft  surface-sheet  was  finally 
removed  and  the  formerly  buried  peneplain  exposed,  some 
of  the  streams  maintained  their  courses  and  still  flowed 
through  water-gaps  in  the  ridges,  while  others,  by  the  system 
of  adjustment  to  rock  structure  discussed  on  a  previous 
page,  were  diverted  to  easier  courses,  leaving  notches,  or 
wind-gaps. 

The  process  by  which  streams  are  inherited  by  one  series 
of  rocks  from  a  higher  series,  is  simulated  in  seme  of  its 
features  by  a  reverse  process.  When  the  roof  of  a  cavern 
crumbles  and  falls  in,  a  subterranean  avenue  of  drainage 
becomes  an  open  channel,  and  the  stream  flowing  through 
it  becomes  a  surface  stream.  Such  a  stream  is  an  inheri- 
tance from  a  lower  series  of  rocks  by  a  higher  series.  If  a 
name  were  desired  for  this  minor  feature  of  the  drainage  of 
certain  regions,  it  might  be  termed  subimposcd.  I  believe, 
however,  that  the  nomenclature  of  geography  should  grow 
slowly  and  spontaneously  and  not  be  forced. 


STREAM  DEVELOPMENT 


W 


MIGRATION   OF   DIVIDES 


When  the  feeding  rivulets  of  two  rivers  flow  in  opposite 
directions  from  the  crest  of  a  mountain  range,  the  line 
separating  them  is  termed  a  water-parting,  or,  more  briefly, 
a  dhnde.  In  general,  then,  a  divide  is  the  common  bound- 
ary between  tvo  adjacent  drainage  systems.  In  the  Rocky 
Mountain  region  we  have  the  "  continental  divide,"  which 
parts  the  waters  flowing  to  the  Atlantic  and  Pacific  respect- 
ively.       "-^^-  -v"^     ■■'.  . :'  -  .  /    ' 

A  divide,  however,  is  not  necessarily  a  mountain  range, 
but  may  be  a  plateau  or  a  plain.  Neither  is  it  a  sharply 
defined  line,  but  may  be  a  broad  surface  and  the  actual  water- 
parting  indefinite.  For  example,  a  portion  of  the  divide  be- 
tween the  waters  flowing  to  Red  River  and  thence  to  Lake 
Winnipeg  and  Hudson  Bay,  and  those  finding  their  way  to 
the  Mississippi,  is  in  a  region  of  mild  relief  and  is  not  only 
broad  but  varies  with  seasonal  changes.  During  certain 
seasons  when  the  streams  are  swollen,  there  is  no  true  divide, 
the  valley  being  flooded  in  such  a  manner  that  steam- 
boats may  pass  from  the  Mississippi  to  the  Red  River,  or 
in  the  reverse  direction.  Thus,  at  times,  direct  river  navi- 
gation is  possible  from  the  Gulf  of  Mexico  to  Hudson  Bay. 

The  divides  between  adjacent  drainage  slopes  and  the 
ridges  between  neighbouring  streams  belonging  to  the  same 
river  system,  although  geographically  of  great  importance 
and  among  the  most  persistent  features  in  the  topography 
of  the  land — since  corrasion  along  them  is  reduced  to  a 
minimum, — are  yet,  like  all  other  elements  in  a  landscape, 
subject  to  change. 


!l 


M  ^  ^ 


PMPi     .M 


248 


RIVERS  OF  NORTH  AMERICA 


i.  -i! 


The  laws  governing  the  migration  of  divides  have  already 
been  briefly  considered  in  discussing  river  piracy,  but  their 
importance  is  sufficient  excuse  for  some  repetition.  The 
slopes  on  the  opposite  sides  of  a  divide,  it  is  safe  to  say,  are 
never  the  same.  The  streams  descending  the  steeper  side 
will  have  the  greater  velocity  and  will  tend  to  deepen  their 
channels  and  to  extend  their  branches  by  backward  cutting 
more  rapidly  than  their  rivals  flowing  down  more  gentle 
slopes,  and  hence  cause  what  is  termed  a  migration  of  the 
divide.  If  other  conditions  are  the  same,  but  the  streams 
flowing  in  one  direction  from  a  divide  have  a  shorter  course 
to  the  sea  than  their  opposite  neighbours,  the  task  before 
them,  in  order  to  cut  down  their  channels  to  baselevel,  will 
be  less,  and  consequently  sooner  accomplished.  Hence,  as 
the  work  of  the  opposite-flowing  stream  progresses,  the 
divide  between  them  will  be  shifted  toward  the  one  that 
works  more  slowly.  The  area  drained  by  the  shorter  stream 
will  be  enlarged  at  the  expense  of  its  less  active  neighbour. 
Again,  if  the  rocks  on  one  side  of  a  divide  are  softer,  or 
more  easily  dissolved  than  those  on  the  opposite  side,  other 
conditions  being  the  same,  the  streams  flowing  over  the 
softer  rock  will  evidently  progress  with  their  task  more 
rapidly  than  those  having  to  cut  down  their  channels  in 
more  resistant  material,  and  hence  will  be  enabled  to 
extend  their  head  branches  more  rapidly  than  their  op- 
ponents and  capture  new  territory.  In  other  words,  the 
divide  will  be  shifted  toward  the  side  where  the  hard  rocks 
occur. 

Should  the  rain-fall  on  one  side  of  a  divide  be  heavier 
than  on  the  opposite  side,  the  streams  on  the  rainy  side  will 


STREAM  DEVELOPMENT 


249 


be  larger  than  on  the  other  side,  and,  in  general,  will  lower 
their  channels  more  rapidly  than  their  weaker  opponents. 
Should  inequality  in  precipitation  be  the  controlling  condi- 
tion, manifestly  the  divide  would  migrate  toward  the  side 
having  the  smaller  rain-fall.  Yet  another  condition  which 
might  cause  the  migration  of  a  divide,  if  not  controlled  by 
other  circumstances,  is  the  structure  of  the  rocks  forming  the 
water-parting,  xf  the  rocks  are  in  layers  and  gently  inclined 
toward  one  drainage  slope,  as  is  frequently  the  case  in 
ridges  due  to  faulting,  the  streams  descending  the  longer 
slope  will  have  to  remove  more  rock  in  order  to  reach  base- 
level  than  the  swifter  streams  flowing  over  the  broken  edges 
of  the  strata  exposed  in  the  steeper  side  of  the  ridge.  In 
nature,  also,  the  conditions  just  postulated  are  usually 
coupled  with  others  which  favour  the  more  rapid  cutting  of 
the  edges  of  the  inclined  beds.  The  declivity  of  a  ridge 
formed  of  inclined  beds  is  usually  steeper  on  the  side  in 
which  the  edges  of  the  strata  are  exposed ;  the  streams 
on  that  side  are  thus  given  a  steeper  grade  and  flow 
more  swiftly  than  their  opponents,  thus  favouring  more 
rapid  corrasion.  If  alternating  hard  and  soft  beds  oc- 
cur, this  again  favours  the  work  of  the  streams  flowing 
over  their  broken  edges,  by  allowing  them  to  remove 
the  exposed  portions  of  the  soft  layers,  thus  undermin- 
ing the  more  resistant  beds  and  favouring  their  removal  by 
sapping.  '  J 

The  Sierra  Nevada  is,  in  the  main,  a  great  monoclinal 
ridge  of  the  character  just  described.  The  strata  dip  west- 
ward and  form  a  long  and  comparatively  gentle  slope  on  that 
side,  but  present  a  bold  escarpment  formed  of  the  broken 


Iff! 


250 


RIVERS  OF  NORTH  AMERICA 


p.  . 

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edges  of  the  strata  upraised  along  a  belt  of  faulting  to  the 
eastward.  The  streams  flowing  'vestward  are  larger  than 
those  descending  the  steep  eastern  slope  for  the  reason  that 
they  drain  greater  areas,  and  also  because  the  rain-fall  on 
the  western  is  more  abundant  than  on  the  eastern  slope. 
But  in  spite  of  these  advantages  the  eastward-flowing 
streams,  having  steeper  gradients,  and  far  less  rock  to  re- 
move in  order  to  cut  to  the  same  depth,  have  been  enabled 
to  extend  their  head-waters  by  backward  corrasion  so  as  to 
cut  through  what  was  formerly  the  divide  at  the  crest  of  the 
range  and  acquire  territory  on  the  western  slope.  The 
water-parting  is  now  west  of  the  topographic  crest-line  of 
the  mountain  and  is  still  migrating  westward.  The  exist- 
ence of  large  glaciers  on  the  higher  portions  of  the  range,  at 
a  comparatively  recent  date,  interfered  with  stream  develop- 
ment, but  did  not  change  the  conditions  so  far  as  the 
westward  migration  of  the  divide  between  the  Pacific  and 
Great  Basin  drainage  is  concerned. 

It  does  not  seem  necessary  to  present  other  arguments  in 
order  to  establish  the  law  that  when  a  ridge  dividing  two 
drainage  systems  is  composed  of  inclined  beds  which  slope 
in  one  direction  rom  its  longer  axis,  other  conditions  being 
the  same,  the  divide  will  migrate,  as  erosion  progresses, 
with  the  slope  of  the  beds. 

The  outline  presented  in  the  last  few  pages  of  the  laws 
governing  the  migration  of  divides,  although  brief,  is  suffi- 
cient to  show  that  the  conditions  entering  into  the  problem 
are  complex.  This  complexity  is  still  further  enhanced 
when  the  influence  of  movements  in  the  earth's  crust  are 
also   brought  into  play.     Although,  as   previously  stated, 


Plate  XIII. 


:}.:::. 


Contour  Map  of  a  Portion  of  llieCatskill  Mountains,  N.  Y.,  Illustrating  River  Piracy. 
(After  N.  H.  Darton.     Topography  by  U.  S.  Geological  Survey.) 

Approximate  scale :  i  inch  =  7000  feet.    Contour  Interval,  20  feet. 


i 

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S  TREA  M  DE  VEL  OP  MEN  T 


251 


divides  are  among  the  most  stable  of  the  geographical 
features  of  the  land,  they  are  continually  changing.  This 
shifting  of  the  position  of  the  lines  of  parting  between  op- 
posing drainage  slopes  is  usually  exceedingly  slow,  but 
under  certain  conditions  may  be  a  comparatively  rapid  pro- 
cess, as  will  be  seen  by  reverting  to  the  discussion  of  the 
origin  of  subsequent  streams,  and  the  manner  in  which  they 
extend  their  channel  by  headward  cutting  so  as  to  cap- 
ture rival  streams  and  divert  their  waters.  This  process 
leads  to  great  and  even  rapid  changes  in  the  positions  of 
divides.  ^ 

Another  illustration  of  the  migration  of  a  divide  may  be 
of  interest  to  the  reader.  In  the  Catskili  Mountains  we 
have  a  table-land  sloping  gently  westward,  but  presenting  a 
bold  escarpment  about  one  thousand  five  hundred  feet  high, 
facing  the  Hudson.  A  portion  of  this  plateau  and  its  east- 
ward-facing escarpment  is  shown  on  the  map  forming  Plate 
XIII.  The  rocks  forming  the  plateau  are  sedimentary  beds 
of  hard  sandstone  and  soft  shale,  which  dip  gently  westward 
and  present  their  broken  edges  in  the  escarpment.  As  has 
been  shown  by  Darton,*  the  regularity  of  the  precipitous 
eastern  border  of  the  plateau  was  broken  by  alcoves  and 
recesses,  inherited  from  a  preceding  geographical  cycle,  and 
in  these  embayments  eastward-flowing  streams  originated. 
Of  these  the  Kaaters  Kill  and  Plaaters  Kill  are  the  best 
examples.  Streams  also  came  into  existence  on  the  gentle 
western  slope  of  the  plateau  and  flowed  westward ;  the  head 

'  N.  H.  Darton,  "  Examples  of  Stream-Robbing  in  the  Catskili  Mountains," 
in  bulletin  of  the  Geological  Society  of  America,  vol.  vii.,  pp.  505-507,  Plate 
xxiii.,  1896. 


it 


Hiiii.,, 


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11 


I 

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» 

r 


252 


RIVERS  OF  NORTH  AMERICA 


branches  of  one  of  these,  Schohap'e  Creek,  are  shown  on  the 
accompanying  map.  The  conditions  are  thus  especially 
favourable  for  the  processes  of  stream  capture  and  the 
migration  of  a  divide,  already  described. 

It  is  evident  from  an  inspection  of  the  map,  that  the 
branches  of  Schoharie  Creek  were  formerly  longer  than  now, 
and  carried  away  the  surplus  water  from  the  plateau  even 
to  the  edge  of  its  eastern  escarpment,  but  the  Kaaters  Kill 
and  Plaaters  Kill  have  been  enabled  to  extend  their  head 
branches  so  as  to  capture  a  considerable  portion  of  the  pre- 
vious western  drainage.  The  divide  has  migrated  westward, 
and  some  of  the  former  branches  of  Schoharie  CreeK  have 
been  diverted.  This  history  is  brought  out  so  graphically 
on  the  accompanying  map  that  further  explanation  seems 
unnecessary. 

One  result  of  the  process  just  conr^idered  is  shown  by  the 
direction  of  flow  of  the  higher  branches  of  the  capturing 
streams.  Normally  the  branches  of  a  stream  join  the  main 
trunk  at  an  acute  angle,  the  flow  in  the  branch  and  in  the 
trunk  near  their  place  of  union  being  in  the  same  general 
direction.  But  in  the  case  of  the  capturing  streams  in- 
stanced above,  their  head  branches  come  in  ai  more  than  a 
right  angle;  the  captured  branches  maintain  the  direction 
they  had  when  flowing  to  Schoharie  Creek,  and  in  general 
flow  westward,  while  the  trunk  streams  to  which  they  are 
now  tributary  flow  eastward.  Such  an  abnormal  arrange- 
ment of  the  branches  of  a  drainage  tree  in  any  region  should 
at  once  suggest  that  a  recent  capture  has  been  made,  but 
yet  rock  textuie  and  other  conditions  might  produce  a 
similar  result. 


STREAM  DEVELOPMENT 


253 


n  on  the 
jpecially 
and    the 

that  the 
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au  even 
ters  Kill 
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the  pre- 
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;eK  have 
iphically 
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;  than  a 

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The  process  of  stream  capture,  so  admirably  illustrated  in 
the  Catskills,  furnishes  an  example  of  one  method  by  which 
fishes,  mollusks,  etc.,  might  be  enabled  to  migrate  from  one 
side  of  a  mountain  range  to  the  other.  The  opening  of 
gap?  in  the  crest  of  a  high  ridge  or  mountain  range  would 
also  facilitate  the  distribution  of  plants  and  arimals  not  de- 
pendent directly  on  streams  for  their  means  of  travel.  Im- 
portant influences  even  on  the  migration  of  peoples  may  be 
traced  to  the  same  cause.' 

r 

'  Books  of  reference : 

Humphreys  and  Abbot.  Physics  and  Hydraulics  of  the  Mississippi. 
War  Department,  Washington,  D.  C,  1861. 

Thomas  Russell.  Meteorology.  Macmillan  &  Co.,  1895.  (Chapters  IX., 
"  Rivers  and  Floods,"  and  X.,  "  River-Stage  Predictions.") 

Park  Morrill.  Floods  of  the  Mississippi  River.  Weather  Bureau,  Wash- 
ington, D.  C,  1397. 


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CHAPTER  Vlil  T,-;   . 

SOME  OF  THE  CHARACTERISTICS  OF  AMERICAN 

RIVERS 

THE  many  details  chat  have  occi'pied  the  reader's  atten- 
tion in  the  preceding  chapter  have  perhaps  diverted 
attention  from  certain  general  conclusions  pertaining  to  the 
lives  of  streams.  A  brief  review  of  the  leading  characteris- 
tics of  a  few  of  the  rivers  of  America  will  possibly  correct 
this  tendency  and  at  the  same  time  afford  an  opportunity  to 
apply  some  of  the  principles  stated,  perhaps  too  empirically, 
in  what  has  gone  before. 

The  initial  slopes  of  large  rivers  must  evidently  be  deter- 
mined by  the  slope  of  the  land  due  to  upheaval.  In  many, 
and  probably  most,  instances,  however,  the  surface  slopes 
that  gave  direction  to  the  youthful  streams  have  been  de- 
formed bv  movements  in  the  rocks  of  the  nature  of  a  tilt- 
ing  of  the  land  over  broad  areas;  again,  the  rocks  have 
been  folded,  or  broken  and  one  side  of  the  fracture  upraised 
above  the  opposite  side,  so  as  to  affect  the  suiface  drainage. 
While  these  changes  were  in  progress  in  many  instances,  the 
streams  have  maintained  their  positions  or  rigiit  of  way  oy 
deepening  their  channels  as  fast  as  the  rocks  were  raised,  or 
by  filling  in  the  depressions  due  to  subsidence;  but  in  other 
instances  the  streams  have  been  reversed  or  given  other  di- 

854 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     255 


/  A 


rections,  owing  to  the  modifications  in  conditions  referred 
to.  The  present  courses  of  even  the  larger  rivers  do  not, 
therefore,  in  themselves,  necessarily  record  the  original 
slope  of  the  land. 

Throughout  the  lives  of  streams  they  have  the  power  of 
extending  their  branches  in  a  manner  analogous  to  the 
growth  of  a  tree  by  the  lengthening  of  its  terminal  twigs. 
This  process,  as  we  have  seen,  leads  to  rivalry  between 
neighbouring  streams,  and  the  shifting  or  migration  of  the 
boundary  line  between  adjacent  drainage  areas.  Climatic 
changes  may  also  favour  the  extension  of  certain  drainage 
areas  and  the  diminution  of  others.  For  these  and  still 
other  reasons,  the  boundaries  of  the  original  slopes  which 
gave  the  large  rivers  their  general  directions  have  been 
greatly  modified  and  in  some  instances  rendered  indeter- 
minate; yet  when  the  general  changes  that  land  areas 
pass  through  and  the  laws  of  stream  development  are 
known,  much  of  the  history  of  a  river  system  can  be 
deciphered.  ■  • 

Some  of  the  modifications  that  have  taken  place  in  the 
various  drainage  areas  of  North  America,  due  to  changes  in 
the  elsvatior  of  the  land,  variation  of  climate,  normal  stream 
development,  etc.,  can  be  recognised  even  in  a  general  view 
of  the  present  distribution  of  the  streams.  Individual 
rivers  furnish  too  small  a  unit  with  which  to  measure  the 
greater  slopes  produced  in  the  surface  o*"  North  America  by 
upheaval,  and  a  better  idea  of  the  character  the  surface  of 
the  continent  would  present,  had  there  been  no  erosion,  can 
be  had  by  considering  the  main  drainage  areas.  It  must  be 
remembered,  however,  that  the  upheavals  which  established 


"mm 


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256 


HI  VERS  OF  NORTH  AMERICA 


the  main  divides  occurred  at  widely  separated  intervals,  and 
that  in  many  instances,  as  in  the  Appalachians,  the  main 
rivers  have  been  persistent  through  more  than  one  geograph- 
ical cycle. 


DRAINAGE  SLOPES 


j'l 


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An  examination  of  any  fairly  good  map  of  North  Amet'ca 
will  show  that  the  continent  is  divided  into  nine  principal 
drainage  slopes.  These  may  be  conveniently  named,  as  has 
been  done  on  the  map  forming  Plate  XIV.,  after  the  water- 
bodies  into  which  their  rivers  discharge.  This  classification 
is,  in  fact,  arbitrary,  and  certain  minor  or  but  Uttle-known 
regions,  as  the  north-east  coast  of  Labrador,  much  of  the 
Arctic  archipelago,  Greenland,  etc.,  are  not  included.  The 
divisions  chosen,  largely  for  the  purpose  of  dissecting  a  vast 
region  into  its  component  parts  for  convenience  of  study, 
are  described  below.  These  descriptions  are  brief,  and 
intended  simply  to  supplement  the  accompanying  map. 

Atlantic  Drainage  Slope. — This  includes  the  land  from 
Florida  to  Nova  Scotia  which  drains  to  the  Atlantic.  The 
principal  rivers  are  the  Alabama,  Savannah,  Roanoke, 
James,  Potomac,  Susquehanna,  Delaware,  Hudson,  Con- 
necticut, Merrimac,  and  St.  John. 

St.  Lawrence  Drainage  Slope. — The  region  draining  to  the 
Great  Lakes  and  Lake  Champlain,  or  directly  to  the  St. 
Lawrence  and  its  tributaries,  is  here  included. 

Hudson  Bay  Drainage  Slope. — This  division  comprises  the 
vast  forested  area  of  low  relief  lying  principally  in  Canada, 
and  draining  through  many  valleys  to  Hudson  Bay. 

Arctic  Drainage  Slope. — Comprising  the  region  north  of 


Plate  XIV. 


K-J^x^ 


tTAturi     »H.t9 


Outline  Map  of  North  America  showing  Draiiia^^e  Slopes 

A— Arctic  DrainaRe  Slope.  Q— Gulf  nt  Mexico  Drainage  Slope 

At.  -Atlantic  DrainaKc  Slope.  Q  B-dre.it  H.isiii  Draimge  Slope. 

B— Hcring  Sea  DrainaBe  Slope.  H— Hudson   Hay  Drainajie  Slop*. 

C— Caribbean  Sea  Drainage  Slope.  Pl'acirtc  Drainage  Slope. 

St.  L— St.  Lawrence  Dra'.nagc  Slope. 


;  f 


pp 


T 


1 1 


I 


i 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     25/ 


the  Hudson  Bay  drainage,  several  of  the  little-known  Arctic 
islands,  the  basin  of  the  Mackenzie,  and  northern  Alaska. 

Bering  Drainage  Slope. — Embracing  the  basins  of  the 
Yukon,  Kuskoquim,  and  several  subordinate  rivers,  which 
■discharge  into  Bering  Sea. 

Pacific  Drainage  Slope. — The  long  and  comparatively  nar- 
row belt  of  country,  which  discharges  its  drainage  to  the 
Pacific,  might  be  subdivided  under  the  general  plan  here 
followed,  but  this  does  not  seem  necessary  here  at  present. 
All  of  the  land  from  the  Aleutian  Islands  to  Panama,  which 
sends  its  contributions  of  surplus  waters  to  the  Pacific,  is  here 
included.  The  principal  rivers  are  the  Copper,  Stikine, 
Fraser,  Columbia,  Sacramento,  and  Colorado. 

Great  Basin  Drainage. — The  arid  region  embracing  the 
eastern  border  of  California,  nearly  all  of  Nevada,  Southern 
Oregon,  and  a  large  part  of  Utah,  which  does  not  send  any 
streams  to  the  ocean,  is  here  included.  Similar  but  sub- 
ordinate interior  basins  in  Mexico  are  at  present  neglected. 

Gulf  Drainage  Slope. — All  of  the  land,  including  the  vast 
hydrographic  basin  of  the  Mississippi,  which  is  drained  by 
streams  flowing  to  the  Gulf  of  Mexico,  is  here  considered  as 
a  single  drainage  slope. 

Caribbean  Drainage  Slope. — The  region  from  Northern 
Yucatan  to  the  junction  of  the  Isthmus  of  Panama  with  the 
South  American  continent,  from  which  streams  flow  to 
the  Caribbean  Sea,  foims  the  most  southern  of  the  several 
drainage  slopes  here  considered. 


The  above  classification,  as  has  been  said,  is  in  part  arbi- 
trary, but  in  its  main  features  is  believed  to  indicate  condi- 


•7 


w 


r 

• 


I 


r  m 


258 


RIVERS  OF  NORTH  AMERICA 


tions  which  a  geographer  finds  it  convenient  to  have  in 
mind. 

The  present  extent  and  r.elationships  of  the  drainage 
slopes  noted  above  are  in  part  due  to  the  movements  in 
the  earth's  crust,  and  to  what  may  be  termed  the  accidental 
inequalities  of  the  original  surface  of  the  land,  but  not  at  a 
single  period.  Each  separate  province  has  its  own  special 
hisiory.  In  part,  also,  the  boundaries  of  the  drainage 
slopes  are  dependent  on  present  climatic  conditions,  as  is 
seen  in  the  Great  Basin  region.  Then,  too,  the  character 
of  the  rocks,  whether  soft  or  hard,  and  the  way  in  which  the 
layers  composing  '  hem  are  inclined,  have  had  a  directing  in- 
fluence on  the  migrations  of  the  dividing  lines  between 
adjacent  drainage  areas.  To  a  marked  extent,  also,  the 
boundary  lines  under  consideration  have  been  shifted  by 
what  is  termed  stream  development,  during  which  one 
stream  extends  its  head-water  branches  so  as  to  capture 
territory  belonging  to  a  neighbouring  stream.  In  several 
great  regions  in  North  America,  the  land  has  been  worn 
down  to  a  peneplain,  and  then  upraised,  thus  making  con- 
spicuous changes  in  the  balance  of  powei  r.inong  the  streams 
of  various  drainage  areas.  Again,  the  divides  between  the 
drainage  slopes  in  the  northern  half  of  the  continent  have 
been  modified  by  glacial  action.  Still  other  complexities  in 
the  histories  of  the  drainage  slopes  as  we  now  find  them 
will  appear  later. 

A  hasty  glance  at  the  great  natural  divisions  of  North 
America,  as  marked  out  by  the  directions  of  drainage,  re- 
veals the  fact  that  the  study  we  have  undertaken  is  a  por- 
tion of  w  long  and  still  mor^  highly  complex  history.     In 


t^ 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS      2$g 


treat i 


ich 


the 


broad  subject 

history  of  the  streams  of  a  continent,  it  is  advisable  to  begin 
with  individual  streams,  and  to  start,  perhaps,  with  even 
their  tiniest  branches,  and  gradually  expand  our  field  of  ob- 
servation. Each  of  the  drainage  slopes  enumerated  has  its 
main  rivers  fed  by  many  branches.  Each  trunk  stream  and 
each  individual  branch,  however  small,  is  an  active  agency 
which  is  engaged  in  producing  changes,  and  each  trunk  and 
branch  of  the  many  drainage  systems  is  modified  in  its 
action  by  climatic,  geological,  and  other  conditions. 


■»!f! 


LEADING  FEATURES  OF  THE  SEVERAL  DRAINAGE  SLOPES 

Ni'tu  Engla7id  Rivers. — The  Connecticut  with  its  charming 
scenery,  the  historic  Merrimac,  the  forest-bordered  Kenne- 
bec, and  many  other  streams  in  the  northern  portion  of 
the  Atlantic  drainage  slope  flow  through  valleys  sunken  in  a 
tilted  peneplain.  The  general  level  to  which  the  hills  rise 
throughout  the  lower  courses  of  these  streams  is  itself  a 
record  of  the  work  of  rivers ;  for  the  land,  during  a  geographic 
cycle  long  since  closed,  was  cut  away  to  near  sea-level  and 
then  bodily  upraised  and  tilted  southward.  The  planation 
by  the  old  streams  was  not  complete,  and  the  remnants  of 
the  uplands  that  were  left  rise  as  mountains  above  the  gen- 
eral level  of  the  plateau  in  which  the  modern  rivers  have 
entrenched  themselves.  The  portions  of  the  plateau  sur- 
face which  were  once  nearly  smooth  have  been  roughened 
by  the  excavation  of  valleys,  leaving  the  spaces  between 
the  streams  in  relief. 

The  rivers  meander  through  rich  bottom-lands  of  their 


ii 


26o 


RIVERS  OF  NORTH  AMERICA 


IT.  r 


!!i 


own  making.  The  borders  of  the  valleys  are  frequently  in 
steps  or  terraces  rising  one  above  another;  each  nearly  flat- 
topped  shelf  furnishes  sites  for  prosperous  farms,  thriving 
villages,  or  populous  cities.  The  terraces  on  the  sides  of 
the  present  valleys  are  formed  of  gravel  and  sand,  and  show 
how  deeply  the  still  older  valleys  were  filled  and  the  pro- 
gress that  has  been  made  in  their  re-excavation. 

Not  only  have  the  valleys  long  and  varied  histories,  but 
each  roaring  cascade  and  musical  rapid,  each  shadowy  pool 
and  placid  reach  of  the  streams  where  the  water  loiters,  and 
each  graceful  bend  have  a  cause  for  their  existence,  and  an 
instructive  and  even  romantic  story  to  tell. 

A  Droivncd  River. — The  noble  Hudson,  in  large  part  an 
arm  of  the  sea,  where  the  tides  rise  and  fall,  divides  a 
mountain  range.  The  early  history  of  the  river  has  not 
been  fully  traced,  but  apparently  it  had  its  course  defined 
before  the  mountains  were  elevated  athwart  its  course,  and 
cut  down  its  channel  as  fast  as  the  land  rose.  Possibly  it 
underwent  a  long  process  of  adjustment  to  geological  condi- 
tions, and  experienced  many  vicissitudes  due  to  changes  of 
level,  and  has  a  far  more  complex  history  than  the  present 
features  of  its  valley  clearly  indicate.  At  a  late  period  it 
flowed  far  beyond  the  site  of  the  great  metropolis  situated 
near  its  present  junction  with  the  sea,  but  a  subsidence  led 
to  the  submerging  of  its  valley  eighty  miles  eastward  of 
Long  Island,  and  transformed  its  upper  course  into  an 
estuary  as  far  as  the  city  of  Troy.  The  Hudson,  in  addition 
to  its  connection  with  American  history  and  the  wonderfully 
attractive  scenes  along  its  course,  has  a  long  and  varied 
experience  to  relate. 


m 


uently  in 
2arly  flat- 
,  thriving 
:  sides  of 
and  show 
the  pro- 

ories,  but 
lowy  pool 
)iters,  and 
:e,  and  an 

^e  part  an 
divides  a 
r  has  not 
ie  defined 
)urse,  and 

ossibly  it 
cal  condi- 
hanges  of 
le  present 

period  it 
situated 
dence  led 

tward  of 
into  an 
addition 

nderfuUy 

id  varied 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     26 1 

Appalachian  Rivers. — The  Delaware,  Susquehanna,  Poto- 
mac, and  James  rise  to  the  westward  of  the  Appalachians 
and  flow  through  the  many  separate  ridges  composing  that 
wonderfully  beautiful  mountain  system.  They  traverse  the 
ridges  in  deep,  narrow  gorges,  known  as  water-gaps,  and 
enter  wide-mouthed  estuaries.  Evidently  the  mountains 
did  not  exist  at  the  time  the  courses  of  the  rivers  werq 
established.  The  long  ridges  separating  the  picturesque 
and  fruitful  valleys  in  the  Appalachian  Mountains  are 
level-topped,  and  rise  in  a  large  number  of  instances  to  the 
same  general  height.  Fill  the  valleys  to  the  level  of  the  in- 
tervening uplands,  and  a  plateau  with  an  even  surface  would 
result.  The  restored  plateau  would  slope  south-eastward, 
and  streams  flowing  down  it  would  cross  the  tilted  rock- 
layers  composing  it  at  right  angles.  The  courses  of  the 
main  streams,  if  once  established  on  such  a  plane,  would  be 
maintained  unless  marked  disturbances  occurred,  and  would 
slowly  deepen  their  channels  and  develop  new  branches. 
For  tens  of  thousands  of  years  the  tireless  streams  would 
work  at  their  task.  The  soft  rocks  would  be  removed  with 
comparative  ease,  leaving  the  hard  layer  in  bold  relief.  The 
result  would  be  a  deeply  dissected  plateau  like  that  of 
eastern  Pennsylvania  or  West  Virginia.  The  filling  in  of 
this  outline  sketch  of  the  origin  of  the  bolder  features  so 
much  admired  by  travellers  over  the  Appalachian  divisions 
of  the  Pennsylvania  or  the  Baltimore  and  Ohio  railroads, 
\/ill  give  a  picture  of  the  geographical  development  of  a 
large  tract  of  rugged  country  embraced  in  the  west-central 
portion  of  the  Atlantic  drainage  slope. 

The  rivers  flowing  eastward  from  the  Appalachians  cross 


.» 


I' 


262 


RIVERS  OF  NORTH  AMERICA 


a  plateau  composed  of  resistant  rocks,  and  on  its  eastern 
border  form  cascades  and  rapids  where  they  descend  to  the 
still  lower  coastal  plain  formed  of  incoherent  strata.  This 
line  of  cascades  and  rapids  extends  from  near  the  Hudson 
to  beyond  the  Savannah,  and  is  frequently  designated  as 
the  "  fall  line."  To  the  west  of  this  line  the  swift -flowing 
rivers  are  shallow,  while  to  the  eastward,  owing  in  part  to  a 
subsidence  of  the  land  and  the  consequent  drowning  of 
their  lower  courses,  their  currents  become  sluggish  and  the 
water  deep.  These  geographical  conditions  resulting  from 
a  long  series  of  changes,  have  had  a  marked  influence  on 
both  the  savage  and  civilised  inhabitants  of  the  Atlantic 
slope.  The  fall  line,  before  the  arri/p.l  of  Europeans,  was 
the  site  of  numerous  Indian  villages.  White  men  coming 
to  America  in  ships  could  ascend  some  of  the  I'vers  to  the 
fall  line,  and  there  found  the  head  of  navigation.  In  order 
to  penetrate  farther  inland  a  new  start  had  to  be  made.  At 
these  same  localities  water  power  was  discovered,  and  man- 
ufactories established.  For  the-"^  and  other  reasons  the  fall 
line  became  a  line  of  cities.  If  we  draw  a  line  on  a  moder- 
ately small-scaled  map  of  the  Atlantic  States,  through  the 
sites  of  Trenton,  Philadelphia,  Baltimore,  Washington, 
Richmond,  Weldon,  Raleigh,  and  Augusta,  it  will  mark- 
out  a  belt  of  the  earth's  crust  which  has  experienced  re- 
peated movements,  and  define  with  considerable  accuracy 
•the  junction  of  the  piedmont  plateau  with  the  coastal  plain, 
and  show  the  position  of  the  fall  line,  v  n;,  _^  , 
\^i  Rivers  of  Glaciated  Lands. — Tens  of  thousands  of  streams 
in  the  northern  part  of  the  Atlantic  drainage  slope  and  in 
the  region  to  the  north  and  west  are  broken  by  cascades 


Plate  XV. 


:s  eastern 
nd  to  the 
ta.  This 
e  Hudson 
gnated  as 
ft -flowing 

1  part  to  a 
owning  of 
h  and  the 
Iting  from 
luence  on 

2  Atlantic 
)eans,  was 
2n  coming 
ers  to  the 

In  order 

lade.     At 

and  man- 

ns  the  fall 

a  moder- 

rough  the 

shington, 

will  mark 

enced  ro- 

accuracy 
stal  plain, 

)f  streams 
pe  and  in 
cascades 


k 


Fig.  a.     The  Columbia  Looking  West  from  White  Salmon,  Washington, 
(rhotograph  by  J.  H.  Valentine.) 


'f  ■« 


Fig.  B.     The  Hudson,  from  West  Point,  New  York. 

(Photograph  by  W.  H.  Rau.) 
^         ILLUSTRATIONS  OF  DROWNED  RIVER-VALLEYS. 


iii- 


fi 


t\i 


i  1/ 


F 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     263 


and  rapids  and  drain  many  thousands  of  lakes.  These 
features,  which  add  a  crowning  charm  to  northern  land- 
scapes, are  almost  entirely  absent  from  the  southern  Appa- 
lachians and  the  Gulf  drainage  slope.  Why  this  striking 
contrast  ?  Why  t  .v^ald  the  streams  from  central  Pennsyl- 
vania to  Labrador  plunge  over  precipices,  or  form  foam- 
ing rapids,  and  the  land  they  drain  be  studded  with  tarns, 
lakes,  and  great  fresh-water  seas;  whi'e  the  streams  in  an 
equally  elevated  and  fully  as  picturesque  region,  but  with 
different  details,  at  the  south,  flow  in  evenly  sloping 
channels  except  on  their  extreme  head-waters,  through  a 
land  that  is  completely  drained  and  in  which  lakes  similar 
to  those  at  the  north  are  absent  ?  The  answer,  thanks  to 
Louis  Agassiz  and  other  students  of  glaciers,  is  that  ice- 
sheets  of  vast  extent  gave  a  new  surface  to  the  land  over 
the  northern  half  of  the  continent,  and  to  a  great  extent 
obliterated  the  valleys  and  stream  channels  made  during  a 
preceding  period  of  mild  climate  and  luxuriant  vegetation. 
Southern  Rivers. — What  charming  pictures  of  placid 
rivers  flowing  between  wooded  and  flower-bedecked  banks, 
softened  and  partially  obscured  perhaps  by  morning  mists, 
enrich  the  memories  of  those  who  have  travelled  in  the 
Carolinas,  Georgia,  and  Alabama!  Whence  the  fascination 
of  these  sleepy  streams,  flowing  through  flat-bottomed  val- 
leys bordered  by  mildly  roughened,  plateau-like  uplands  ? 
What  has  subdued  the  broader  features  of  the  landscape  in 
a  region  where  every  river  bank  reveals  folded  and  con- 
torted rocks,  similar  to  those  in  the  neighbouring  mountains? 
The  geographer  sees  evidence  at  every  turn  that  mountains 
once  existed,  but  that  they  have  been  removed.     A  cycle 


Ml 


w^^ 


V 


'  hi 


I   ' 


264 


RIVEHS  OF  NORTH  AMERICA 


of  geographical  history  has  run  its  course,  and  a  new  cycle 
has  been  initiated  at  a  comparatively  recent  date.  The 
land,  once  rugged  and  mountainous,  has  been  carved  away, 
with  the  exception  of  certain  island-like  remnants  or  mo- 
nadnucks,  to  a  uniform  level, — the  horizon  of  the  sea, — then 
moderately  upraised  and  the  surface  of  the  gently  tilted 
peneplain  channelled  by  streams. 

Alluvial  Rivers. — In  the  low-lying  regions  bordering  the 
Gulf  of  Mexico,  the  traveller  finds  sluggish  rivers  flowing 
through  broad  valleys  which  are  flooded  each  springtime, 
or  in  early  summer,  when  the  snow  melts  on  the  mountains 
to  the  north  and  west.  Every  stn  is  margined  by  natural 
embankments  or  levees,  which,  when  not  modified  by  human 
agency,  are  built  higher  during  each  succeeding  flood. 
With  the  raising  of  the  levees  the  valley  bottoms  are  in- 
undated and  a  layer  of  fine  rich  soil  deposited  over  them. 
Evidently  the  streams,  instead  of  deepening  their  channels 
after  the  manner  of  the  swift-flowing  rivers  of  New  Eng- 
land, are  filling  in  previously  formed  depressions  or  making 
new  lands  along  the  Gulf  coast.  It  is  plain  to  be  seen  that 
the  laying  down  and  not  the  removal  of  material  is  in  pro- 
gress over  vast  areas,  and  that  rich  land-  *  •  leing  formed, 
on  which  rice  and  sugar-cane  can  be  cul'i,  -  .  No  mount- 
ains or  hills  are  in  sight.  The  land  is  flat,  -^c  \  nout  valleys, 
and  except  where  fields  have  been  cleared,  is  clothed  with 
tangled  vegetation.  Cypress  tree^,  and  white-trunked  cotton- 
woods  lean  far  over  the  yellow  waters,  their  branches  fes- 
tooned with  trailing  vines  and  pendant  lichens.  Much  that 
is  interesting  concerning  the  manner  in  which  streams  fill  in 
their  valleys  during  certain  stages  in  their  histo  ies,  and  the 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     26$ 

way  in  which  new  areas  are  reclaimed  from  the  sea,  may 
here  be  studied.  The  process  of  stream  excavation,  most 
active  where  the  land  is  high,  the  gradients  steep,  and  the 
waters  swift,  here  finds  its  complement  where  the  land  is 
low,  the  gradients  gentle,  and  ';he  waters  sluggish.  The 
rivers,  although  large,  are  unable  to  bear  along  the  sediment 
delivered  to  them  in  theii  swifter  upper  courses,  and  it  is 
laid  aside  in  flood-plains  and  deltas  to  rest  until  the  facilities 
for  transportation  are  mere  favourable.  The  southern  por- 
tion of  the  Gulf  drainage  slope  furnishes  abundant  illustra- 
tions of  the  fact  that  an  important  part  of  the  work  of 
streams  consists  in  depositing  material  and  the  aggrading 
or  filling  in  of  their  channels  and  valleys.  The  streams  of 
New  England  and  of  the  Gulf  region,  although  presenting 
marked  contrasts,  are  not  essentially  different,  but  have 
reached  different  stages  in  their  life  histories,  and  have  felt 
the  influence  of  diverse  modifying  conditions. 

T/ic  Mississippi. — Some  of  the  characteristic  features  of 
the  lower  portion  of  this  the  greatest  of  all  the  rivers  of 
North  America  have  be^n  included  in  the  glance  we  have 
just  given,  which  i?  all  that  space  will  allow,  to  the  alluvial 
rivers  of  the  Gulf  coast.  The  great  importance  of  the  Mis- 
sissippi as  a  highway  of  commerce  and  of  civilisation,  the 
vast  agricultural  interests  of  its  basin,  and  the  numerous 
illustrations  of  the  behaviour  of  streams  under  ividely  con- 
trasted conditions  and  in  various  stages  of  development  fur- 
nished by  it,  tempt  the  student  of  American  geography  to 
visit  it:  banks  again  and  again,  and  to  long  to  explore  its 
entire  extent  with  the  searchlight  of  modern  geographical 
methods. 


266 


RIVERS  OF  NORTH  AMERICA 


k 
t 
c 

I 


The  source  of  the  central  trunk  of  the  Mississippi  is  in 
Lake  Itasca,  but,  as  is  well  known,  a  great  branch  of  the 
drainage  tree,  the  Missouri,  far  overtops  the  summit  of  its 
central  stem.  To  the  geographer  the  true  source  of  the 
Mississippi  is  at  the  as-yet-unknown  fountainhead  of  the 
Missouri.  The  waters  forming  the  Missouri  are  supplied  in 
part  by  the  hot  springs  and  wonderful  geysers  of  the  Yellow- 
stone Park,  and  in  part  by  snow  banks  and  small  glaciers  in 
the  more  elevated  valleys  and  amphitheatres  of  the  Rocky 
Mountains,  in  Idaho  and  Montana.  To  a  small  extent  the 
waters  of  the  great  river  come  from  Canadian  territory. 
The  countless  lakes  of  Minnesota  and  Wisconsin  expand 
like  leaves  on  the  terminal  twigs  of  the  central  trunk.  The 
head-waters  of  the  Ohio,  the  largest  branch  of  the  Missis- 
sippi which  joins  it  from  the  east,  rise  on  the  western  slope 
of  the  Appalachian  uplift  in  West  Virginia  and  Pennsyl- 
vania. A  small  portion  of  south-western  New  York  is  also 
included  in  the  Ohio  drainage  basin,  ihe  distance  in  a 
straight  line  from  the  head-waters  of  the  Ohio  north-west- 
ward to  the  source  of  the  Missouri  is  over  eighteen  hundred 
miles.  From  the  mouth  of  the  Mississippi  along  its  general 
course  to  the  continental  divide,  which  limits  its  drainage 
basin  on  the  north-west,  is  about  twenty-five  hundred  miles, 
but  including  all  of  the  windings  of  the  river,  the  actual 
distance  that  the  waters  falling  on  the  mountains  of  Mon- 
tana have  to  tra^^cl  in  order  to  reach  th-^  sea,  is  more  than 
four  thousand  miles.  The  entire  area  drained  by  the 
"  Father  of  Waters"  is  about  1,240,000  square  miles,  or 
nearly  one-third  of  the  United  States,  exclusive  of  Alaska. 

Additional   statistics   concerning   the    Mississippi,  taken 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     267 

from  the  report  of  Humphreys  and  Abbot  and  from  a  recent 
report  on  the  floods  of  that  river,  published  by  the  Weather 
Bureau,'  are  here  inserted  : 

Average  annual  precipitation  over  the  entire  basin 29.8  inches. 

Annual  discharge 785, 190,000,000  cubic  yards 

Ratio  of  rain-fall  to  discharge 0.25. 

Mean  discharge  per  second 75,ooo  cubic  yards. 

That  the  Mississippi  was  in  existence  previous  to  the 
Glacial  epoch  is  abundantly  proven  by  the  change  that  oc- 
curs in  its  valley  when  traced  across  the  southern  limit  of 
the  ice  invasion,  which  intersects  its  course  between  the 
mouth  of  the  Missouri  and  the  Ohio.  Southward  of  the 
glacial  boundary  it  flows  through  a  broad  valley  bounded  by 
bluffs.  The  vast  flood-plain,  varying  in  width  from  five  to 
eighty  miles,  lies  from  three  to  five  hundred  feet  below  the 
general  level  of  the  bordering  uplands.  The  contour  of 
the  hard-rock  bottom  of  the  valley  is  but  imperfectly  known, 
but  the  records  of  wells  and  borings  show  that  an  ancient 
valley  has  been  filled  with  alluvium  to  a  depth  of  at  least 
one  or  two  hundred  feet  in  its  northern  part  and  to  an  in- 
creasing depth  southward.  For  about  a  thousand  miles 
northward  from  the  mouth  of  the  river  no  hard  rock  appears 
in  its  bed,  and  cataracts  are  absent.  The  conspicuous  bluffs 
of  light-coloured  clay-like  material  termed  loess,  bordering 
the  flood-plain  in  many  places,  mark  the  borders  of  an  inner 
valley,  excavated  in  the  material  which  formerly  occupied 
the  older  and  broader  valley  from  side  to  side.  The  great 
outer  valley,  eroded  in  !irge  part  through  nearly  horizontal 
beds  of  limestone,  is  a  record  of  the  preglacial  work  of  the 

'  Park  Morrill,  floodi-  of  the  Mississippi.     Washington,  1897. 


h!  ifi 


268 


RIVERS  OF  NORTH  AMERICA 


F 


•:i 


;i:l 


t 


1  'I 


river;  the  inner  valley,  formed  by  the  removal  of  soft  inco- 
herent loess  and  sand,  is  ot  postglacial  origin. 

North  of  the  glacial  boundary,  the  river,  throughout  much 
of  its  course,  flows  through  a  comparatively  narrow,  steep- 
sided  valley,  bordered  by  precipitous  bluffs  of  hard  rock, 
an-'  in  places  the  waters  rush  in  foaming  rapids  or  plunge 
over  ledges  of  limestone.  These  narrow  reaches  have  all 
the  characteristics  of  young  streams.  At  other  times  the 
valley  broadens  and  its  crumbling  sides  are  crowned  by 
towers  and  pinnacles  of  rock  which  bear  every  indication 
of  long  exposure  to  the  air.  These  evidences  of  old  age 
occur  in  what  is  known  as  the  driftless  area  of  Wisconsin 
and  Minnesota,  where  an  island-like  area,  measuring  some 
ten  thousand  square  miles,  existed  in  the  former  ice-sheets. 

In  some  instances  north  of  the  glacial  boundary,  where 
the  river  flows  through  a  narrow,  rock-cut  valley,  deeper  and 
broader  channels  adjacent  to  it  but  now  filled  with  glacial 
debris,  show  that  the  river  was  turned  from  its  preglacial 
course,  when  reborn  after  the  ice  invasion.  In  such  in- 
stances, the  greater  size  and  depth  of  the  old  channels,  in 
comparison  with  their  modern  representatives,  bear  evidence 
that  the  river,  previous  to  the  advent  of  the  glaciers,  had  a 
greater  length  of  time  in  which  to  carry  on  its  appointed 
task,  or  else  worked  with  greater  energy  than  since  the 
glaciers  vanished.  Many  considerations  tend  to  establish 
the  former  of  these  suggestions.  The  Mississippi  was  an 
aged  stream  before  the  great  climatic  change  which  per- 
mitted of  the  extension  of  glaciers  from  the  north  into  its 
drainage  basin. 

The  main  channel  of  the  Ohio  belongs  to  the  extensive 


Plate  XVI. 


:e 


pcr- 
ito  its 


Fig.  a.     An  Aggraded  Valley  near  Fort  Wingate,  New  Mexico. 
Illustrative  of  the  filling  of  valleys  in  arid  regions ;  the  cliffs  are  of  Triassic  sandstone. 


^■'  iF." 


Fig.  B.     Water-Gaps  Cut  l)y  the  I'otomac  through  Two  Kidges  of  Hard  Rock, 

near  Harper's  Ferry,  \V.  Va. 

The  point  of  view  is  on  the  Shenandoah  peneplain  ;  the  Potomac  Hows  thriiii((h  a  steep-sided 
trench  nhout  ii-,  f~'  >  i1eep,  sunken  in  this  peneplain.  Loudoun  Heights  on  the  left  and 
M.iryland  Heights  on  (he  rixht  in  the  background. 


'■,1 

M 


•■yw.i 


i  i; 


i 


|i: 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     269 


system  of  branching  valleys  formed  by  the  sinking  into  the 
rocks  of  the  preglacial  Mississippi  drainage,  but  its  upper 
portion  was  deeply  buried  by  ice  during  the  height  of  the 
Glacial  epoch.  When  the  glaciers  finally  melted,  the  re- 
born streams  found  their  channels  blocked,  and  in  many 
instances  were  turned  from  their  former  courses  in  the  same 
manner  as  in  the  case  of  the  streams  of  Wisconsin  and 
Minnesota. 

When  the  Laurentian  glacier  retreated  to  the  northward 
of  the  height-of-land  now  dividing  the  streams  which  feed 
the  Mississippi  from  those  flowing  to  Hudson  Bay  and  the 
Great  Lakes,  several  lakes  came  into  existence,  which  were 
retained  on  their  northern  margins  by  the  face  of  the  re- 
treating glacier,  and  supplied  southward-flowing  streams. 
One  of  these  glacier-dammed  lakes,  named  in  honour  of 
Louis  Agassi/,  occupied  what  is  now  the  valley  of  Red  River 
and  the  Winnipeg  basin,  and  supplied  River  Warren  which 
flowed  to  the  Mississippi.  Another  similar  lake  was  found 
in  the  western  part  of  the  present  drainage  basin  of  Lake 
Superior,  and  had  its  outlet  a  few  miles  west  of  the  site  of 
the  city  of  Duluth.  Other  lakes  in  this  same  category  oc- 
cupied the  southern  part  of  the  basin  of  Lake  Michigan,  and 
the  western  part  of  the  Eric  basin ;  the  former  discharged  in- 
to the  Mississippi  through  the  channel  now  being  converted 
into  a  canal,  just  west  of  Chicago,  and  the  latter  flowed 
through  the  valley  now  occupied  by  the  Wabash  to  the  west 
of  Fort  Wayne,  Indiana.      __^  i.-^; .-  ^  -,  - -- - -«- 

During  the  time  the  Winnipeg  and  Laurentian  basins 
were  sending  their  surplus  waters  southward  to  the  Mis- 
sissippi, not  only  was  the  run-off  from  the  land  probably 


,,'?•; 


t 

I 

i 


r  \ 


I'i 


I;' 


270 


RIVERS  OF  NORTH  AMERICA 


greater  than  now  on  account  of  heavier  rain-fall,  but  the 
vast  snow-  and  ice-sheet  which  covered  Canada  was  melting, 
and  all  the  stream  channels  leading  away  from  't  were 
flooded.  The  volume  of  water  contributed  in  these  several 
ways  and  flowing  through  the  Mississippi  valley  to  the  Gulf 
of  Mexico  must  at  all  seasons  have  been  far  in  excess  of  the 
greatest  of  the  modern  floods.  It  has  been  estimated  by 
James  E.  Todd  '  that  the  Mississippi,  during  the  geological 
springtime  following  the  great  winter  known  as  the  Glacial 
epoch,  carried  annually  from  eleven  to  twenty  times  the 
volume  of  water  now  reaching  the  Gulf  of  Mexico  through 
the  same  channel  in  a  single  year.  Whether  the  current  of 
the  river  during  this  great  flood  stage  was  vastly  increased 
or  not,  depends  on  the  former  elevation  of  the  land.  These 
are  reasons  for  believing  that  the  region  occupied  by  ice 
was  depressed  several  hundred  feet  below  its  present  posi- 
tion, and  that  the  gradient  of  the  Mississippi  was  much  less 
than  at  present.  The  expanded  rivers  then  resembled  a 
great  sea  in  which  the  loess  and  other  similar  deposits  now 
occupying  the  greater  Mississippi  valley  were  spread  out. 

The  long  preglacial  history  of  the  Mississippi,  the  many 
changes  impressed  by  the  glaciers  directly  on  its  tributaries 
from  the  Appalachians  to  the  crest  of  the  Rocky  Mount- 
ains, and  indirectly,  owing  to  the  vastly  increased  water- 
supply,  on  the  character  of  the  river  all  the  way  to  the 
Gulf,  make  it  a  most  instructive  subject  for  study.  An  ad- 
ditional chapter  in  the  life  of  the  river  is  supplied  by  a 
modern  submergence  which  allowed  the  sea  to  reach  to  the 
mouth  of  the  Ohio,  and  of  still  later  re-elevation.     The  ac- 

■  Geological  Survey  of  Missouri,  1896,  vol.  x.,  p.  203. 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     271 


,  but  the 

1  melting, 

I   't  were 

5e  several 

the  Gulf 

;ss  of  the 

nated  by 

geological 

le  Glacial 

imes  the 

through 

urrent  of 

increased 

.     These 

d  by  ice 

ent  posi- 

luch  less 

mbled  a 

(sits  now 

d  out. 

e  many 

Dutaries 

Mount- 

water- 

to  the 

An  ad- 

by  a 

to  the 

The  ac- 


d 


cidents,  as  they  have  been  termed,  in  the  normal  develop- 
ment of  streams,  due  to  climatic  changes  and  to  movements 
in  the  earth's  crust,  thus  find  numerous  and  graphic 
illustration  in  the  Mississippi  Valley.  Until  the  entire 
basin,  however,  has  been  examined  as  a  unit,  disregarding 
political  boundaries,  even  a  satisfactory  outline  of  its  entire 
geographical  history  cannot  be  written. 

Another  phase  of  the  wonderful  story  of  the  Mississippi 
deals  with  its  influence  on  the  early  explorations  of  the  in-  : 
trepid  emissaries  of  Spain  and  France,  and  the  final  con- 
quests of  its  basin  by  the  English,  the  vast  agricultural 
importance  of  its  rich  lands,  and  its  value  as  an  avenue  of 
commerce ;  but  the  influence  of  geographical  history  on 
human  events  pertains  more  properly  to  the  domain  of  the 
historian,  and  cannot  be  treated  even  briefly  at  this  time. 

Canyon  Rivers. — The  Colorado  River,  rising  in  the  mount- 
ains of  Colorado,  Wyoming,  and  Utah,  and  flowing  through 
a  high  and  for  the  mo,t  part  arid  table-land,  has  carved  in 
the  solid  rocks  the  most  magnificent  canyon  that  has  yet 
been  studied.  The  river,  with  its  load  of  sand  and  mud, 
has  been  able  to  deepen  its  channel  more  rapidly  than  its 
bounding  walls  have  been  lowered  by  rain,  rills,  and  other 
destructive  agencies.  The  result  is  a  steep-walled  trench  of 
such  stupendous  proportions  that  when  its  sides  are  seen 
from  below  they  appear  to  be  towering  mountain  ranges. 
The  tributaries  of  the  Colorado  have  also  deepened  their 
channels  at  approximately  the  same  rate  as  the  main  stream 
has  excavateu  its  canyon.  A  great  river  with  many 
branches  has  thus  been  sunken  into  the  rocks,  to  the  depili, 
over  a  vast  area,  of  about  one  mile.     Between  the  larger 


!ii  i 


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r 


t" 
i 

I 
» 

i 


r 


272 


HI  VERS  OF  NORTH  AMERICA 


Streams  there  are  flat-topped  table-lands,  remnants  of  the 
great  plateau  across  which  the  Colorado  flowed  in  its  infancy. 
The  plateau  has  been  slowly  elevated,  while  the  s.  nd- 
charged  streams,  acting  like  saws,  have  dissected  it. 
Throughout  a  region  tens  of  thousands  of  square  miles  in 
area,  every  stream  is  in  the  bottom  of  a  profound  gorge  of 
its  own  making.  The  remnants  of  the  plateau  between  the 
canyons  are  waterless  and  desert.  x   , 

The  Colorado  throughout  a  large  part  of  its  course  flows 
through  a  canyon  that  is  from  four  to  six  thousand  feet 
deep.  The  canyon  walls  are,  for  the  most  part,  of  horizon- 
tally bedded  rocks  of  many  tones  and  tints,  and  various 
degrees  of  hardness.  Weathering  has  increased  the  variety 
of  colours,  and  rendered  them  more  brilliant  than  they  are 
in  the  unchanged  rocks.  T  \\n  and  wind  have  sculptured 
the  cliffs  so  as  to  give  them  me  greatest  imaginable  variety 
of  forms.  The  most  vivid  dream-pictures  of  gorgeous 
Oriental  architecture  fail  to  rival  the  temple-  and  cathedral- 
like forms,  incrusted  with  harmoniously  tinted  decorations, 
which   overshadow   the   Colorado   for  hundreds   of  miles. 


Fig.  23.  Cross-Profile  of  the  Canyon  of  the  Colorado.    (After  W.  H.  Holmes.) 
,    Vertical  and  horizontal  scale  (he  same  :  one  inch  =  6375  feet. 

There  is  nothing  of  the  same  class  in  the  whole  world,  so 
far  as  is  known,  to  compare  either  in  extent  and  height^  in 
richness  of  colour,  or  in  variety  and  intricacy  of  detail  with 


SOME   CHARACTEklSriCS  OF  AMERICAN  RIVERS     273 


nts  of  the 
ts  infancy, 
the  s.  nd- 
sected  it. 
t  miles  in 
d  gorge  of 
tween  the 

lurse  flows 
I  sand  feet 
if  horizon- 
id  various 
he  variety 
n  they  are 
jculptured 
lie  variety 

gorgeous 
:athedral- 

orations, 
f  miles. 


I.  Hulmes.) 


:et. 


vorld,  so 
eight)  in 
tail  with 


the  canyon  walls  in  the  southern  portion  of  the  Pacific 
drainage  slope.  The  canyon  of  the  Colorado  is  not  an  even- 
sided  canal,  but  a  great  valley  some  fifteen  or  more  miles 
across.  In  the  bottom  of  this  greater  canyon,  as  may  be 
seen  from  the  accompanying  illustration,  Plate  XVII.,  re- 
produced from  a  drawing  by  W.  H.  Holmes,  one  of  the  few 
artists  who  are  true  to  nature,  is  sunken  a  much  narrower 
and  deeper  inner  canyon.  The  reader  will  be  able  to  read 
in  this  picture  some  of  the  leading  events  in  the  geographi- 
cal history  of  the  region  of  the  Great  Plateau.  This  outer 
canyon  is  clearly  the  record  of  a  time  when  the  land  was 
some  four  thousand  feet  lower  than  now,  and  remained  at 
that  horizon  for  tens  of  thousands  of  years,  while  the  river 
cut  down  its  channel  to  baselevel  and  by  lateral  corrasion 
broadened  its  valley.  The  climate,  at  least  near  the  close 
of  this  long  period  of  uninterrupted  work,  was  arid,  as  is 
shown  by  the  precipitous  character  of  the  cliffs  bordering 
the  valley  that  was  excavated.  The  river  meandered  in 
broad  curves  over  the  nearly  level  bottom  of  its  valley,  and 
when  the  land  was  again  raised  maintained  its  winding 
course,  and  owing  to  renewed  energy,  due  to  greater  ve- 
locity on  account  of  an  increase  in  gradient,  again  began  the 
task  of  corrading  to  baselevel.  This  new  task  imposed  upon 
the  river  is  not  yet  completed.  The  waters  still  flow  swiftly, 
and  vertical  corrasion  is  still  in  excess  of  lateral  wear  and  of 
weathering.  The  precipitous  character  of  the  clifTs  border- 
ing the  inner  gorge,  and  the  details  in  their  sculpture,  indi- 
cate that  the  climate  has  had  its  present  characteristics 
throughout  the  greater  part,  and  probably  the  whole,  of  the 

time  since  the  energy  of  the  river  was  renewed. 
18 


274 


RIVERS  OF  NORTH  AMERICA 


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c 

I 


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I'  ; 
i-il 


The  -.vails  of  the  canyon  of  the  Colorado  are  not  even- 
surfaced  precipices,  but  on  either  side  of  the  river  are  but- 
tressed by  outstanding  ridges  and  retaining  walls,  with  many 
lateral  branches.  Everywhere  there  are  towers  and  pin- 
nacles, as  well  as  innumerable  alcoves  and  recesses.  The 
main  buttresses  extend  out  for  miles  from  the  brink  of  the 
gorge  and  partially  fill  the  profound  chasm  into  which  they 
descend.  From  within  the  purple  depth  of  the  canyon  rise 
wondrous  temple-like  forms  as  gorgeous  in  colour  and  as 
rich  in  fretwork  and  arabesque  as  a  Moorish  palace.  These 
shrines  for  Nature-worship,  although  minor  features  in  the 
sublime  panorama,  tower  as  far  above  the  shining  stream 
flowing  past  their  bases  as  the  summit  of  Mount  Washing- 
ton rises  above  the  sea. 

As  an  illustration  of  the  endless  variety,  both  in  form  and 
colour,  of  architectural  forms  that  Nature  c^n  sculpture  from 
an  uptciised  block  of  the  earth's  crust  under  certain  condi- 
tions of  climate  and  rock  texture,  the  Colorado  region  is 
unrivalled.  The  student  of  earth-forms  there  finds  many 
illustrations  of  the  various  phases  that  an  upraised  region 
passes  through,  in  what  may  be  termed  its  youth.  In  a  re- 
view of  the  life  histories  of  rivers,  the  Colorado  furnishes  an 
example  of  a  stream  yet  young,  so  far  as  its  advance  in  its 
appointed  task  is  concerned,  but  one  which,  owing  to  un- 
usual opportunities,  has  surpassed  many  older  but  less 
favoured  sticams  in  the  magnificence  of  the  results  accom- 
plished. The  Colorado  is  not  only  a  young  stream,  but  has 
been  termed  a  precocious  youth,  its  success,  however,  in 
producing  worderful  scenery  of  a  novel  type,  lies  not  so 
much  in  the  amount  of  work  performed,  as  in  the  fact  that 


lot  even- 
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and  pin- 
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Washing- 
form  and 
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lids  many 
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In  a  re- 
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SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     27$ 


alh 


th( 


destructive  agencies  have  spared  the  canyon 

stream   entrenched   itself.      The  climate   is  arid,    and  the 

wasting  of  the  cliffs  consequently  retarded. 

Stupendous  as  are  the  results  achieved  by  the  Colorado, 
and  wonderfully  impressive  as  is  the  scenery  along  its 
course,  the  amount  of  work  it  has  done — that  is,  the  num- 
ber of  cubic  miles  of  rock  removed — is  small  in  compari- 
son with  what  has  been  accomplished  in  many  regions  of 
mild  relief,  where  rivers  in  their  old  age  flow  sluggishly 
over  a  plain  from  which  they  have  removed  nearly  every 
vestige  of  a  former  mountain  range.  The  region  of  great 
plateaus  drained  by  the  Colorado  will,  under  the  tiction  of 
the  agencies  now  in  operation,  be  reduced  to  such  a  plain, 
unless  future  upheaval  again  renews  the  youth  of  the  river. 

Sierra  Nevada  Rivers. — The  numerous  brigiit,  leaping 
rivers  of  the  Sierra  Nevadas,  flowing  through  valleys  three 
to  four  thousand  feet  deep  and  overshadowed  by  pine- 
clothed  mountains,  suggest  many  questions  in  reference 
especially  to  the  influence  of  rock  texture,  changes  in  eleva- 
tion and  glaciation  on  stream  erosion,  and  on  the  origin  and 
developm'int  of  topographic  forms.  The  valleys  are  nar- 
row, with  usually  little  if  .^ny  bottom-land.  The  rivers  are 
swift  and  strong  and  carry  along  with  ease  all  of  the  debris 
delivered  to  them  by  the  bordering  slopes  and  tributaries. 
Not  only  do  they  bear  away  all  of  the  fine  material  that 
reaches  them,  but  in  times  of  high  water  roll  along  large 
boulders,  and  yet  their  capacity  to  transport  is  not  satisfied, 
and  they  are  clear,  limjiid,  joyous  streams  during  a  large  part 
of  the  year.  The  conditions  are  there  the  reverse  of  what  is 
so  manifest  in  the  rivers  of  the  Gulf  States,  where  previously 


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€ 

4 


276 


A'/r/:A'S  OF  KOKTII  AMERICA 


eroded  valleys  are  being  filled  and  broad  areas  of  new 
land  have  been  formed.  In  the  Sierra  Nevadas  the  streams 
are  all  at  work  at  the  task  of  deepening  their  channels; 
the  stage  in  their  lives  when  they  will  broaden  their  val- 
leys more  rapidly  than  they  deepen  them  has  not  been 
reached.  The  Tuolumne,  King,  Truckee,  and  many  other 
rivers  are  not  only  young,  but  are  still  broken  by  cataracts 
aiul  rapids,  and  in  many  instances  are  supplied  in  part  by 
the  overflow  of  lakes.  These  are  plain  evidences  of  youth. 
A  little  study  shows  one,  however,  thit  the  tireless  activity 
of  the  streams  is  largely  due  to  a  recent  uplifting  of  the 
mountains,  which  has  given  them  steeper  slopes,  and  that 
the  presence  of  waterfalls  and  lakes  along  their  courses  is  in 
many  instances  due  to  the  former  existence  of  great  snow- 
fields  and  magnificent  glaciers  in  all  of  the  higher  valleys. 
The  energy  with  which  the  streams  are  working  is  thus  seen 
to  be  due  to  a  revival  of  activity,  or  a  rejuvenation,  rather 
than  to  actual  youthfulness. 

The  westward-flowing  streams  from  the  Sierra  Nevadas 
experience  a  sudden  change  on  leaving  the  mountains  and 
entering  the  flat-bottomed  valleys  where  they  unite  to  form 
the  San  Joaquin  and  Sacramento.  With  loss  of  grade  the 
waters  flow  less  rapidly,  and  their  burdens  are  dropped. 
Deposition  and  aggrading  are  then  the  rule  in.stead  of  abra- 
sion and  valley-deepening.  Borings  made  in  the  bottom  of 
the  broad,  nearly  level-floored  valley  of  California,  show 
that  a  great  depression  between  the  Sierra  Nevada  and 
Coast  mountains  has  been  filled  to  a  depth  of  many  hun- 
dreds of  feet.  Much  of  this  filling  is  due  to  the  deposition 
of  material  swept  out  of  the  bordering  mountains  in  order 


SOME   C//AA'/1Cr/CA'/sr/CS  OF  AMERICAN  RIVERS     277 


if  new 
streams 
an  n  els; 
t\x  val- 
)t  been 
y  other 
itaracts 
part  by 
youth, 
activity 
;  of  the 
nd  that 
ses  is  in 
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lus  seen 
rather 

^cvadas 
ins  and 
to  form 
ade  the 
ropped. 
of  abra- 
ttom  of 
I,  show 

da  and 
ly  hun- 
:)osition 

n  order 


to  form  the  gorges  and  canyons  which  }^ive  them  much  of 
their  diversity  and  beauty.  Conditions  similar  to  those 
already  noted  on  the  Atlantic  coast,  where  a  subsidence  of 
the  land  has  transforiucd  tlic  river  valleys  into  estuaries, 
ar  .igain  manifest  on  the  western  border  of  the  continent. 
The  story  of  stream  development  and  of  changes  in  the  re- 
lief of  the  land  on  the  Pacific  coast,  due  to  the  upheaval  of 
the  land,  is  supplemented  by  the  effects  of  subsidence  on 
the  geography  where  land  and  ocean  meet.  The  bay  of 
San  Francisco  and  its  outlet  tlirough  the  Golden  Gate  show 
that  valleys  have  been  drowned,  owing  to  a  downward 
movement  of  the  land.  Surveys  of  the  sea  bottom  adjacent 
to  the  present  coast-line  reveal  the  fact  that  former  river 
courses  may,  in  some  instances,  be  traced  over  the  conti- 
nental border  now  depressed  beneath  the  Pacific,  in  the 
same  manner  that  soundings  have  demonstrated  a  former 
.seaward  extension  of  the  Hudson,  St.  Lawrence,  and  other 
.streams  of  the  Atlantic  slope. 

It  seems  scarcely  necessary  to  mention,  so  obvious  is  it, 
the  intimate  relation  between  geographical  history  and 
human  activities,  illustrated  by  the  origin  and  marvellous 
growth  of  the  metropolis  of  the  Pacific  coi  st  on  the  border 
of  a  partially  submerged  river  valley.  The  magnificent  bay 
of  San  Francisco,  one  of  the  very  finest  harbours  in  the 
world,  is  a  direct  result  of  a  long  series  of  geographical 
c'langcs.  The  subsidence  which  converted  a  portion  of  the 
valley  of  the  Sacramento  into  an  arm  of  the  sea  has  had  a 
direct  and  far-reaching  influence  not  only  on  the  lives  of 
millions  of  people,  but  on  the  building  of  a  nation.  The 
future  greatness  of  San   Francisco,  assumed  by  her  com- 


E 


I 


Hi' 


278 


RIVERS  OF  NORTH  AMERICA 


manding  geographical  position,  will  make  her  an  important 
factor  in  the  spread  of  civilisation,  not  in  America  alone,  but 
in  the  countries  bordering  the  distant  shores  of  the  Pacific. 

"  Where  Rolls  the  Oregon.'' — The  Columbia  and  its  main 
tributary,  the  Snake,  rise  in  the  Rocky  Mountains,  and  flow 
across  a  region  of  small  rain-fall,  thus  simulating  some  of  the 
main  conditions  which  have  influenced  the  history  of  Colo- 
rado River.  Snake  River  crosses  a  basaltic  plateau  and  has 
excavated  a  magnificent  canyon.  Although  inferior  in  the 
richness  of  its  colouring  and  the  profusion  of  details  in  its 
sculptured  cliffs,  it  is  comparable  in  many  ways  with  the 
Grand  Canyon  of  the  Colorado.  The  walls  of  Snake  River 
canyon  are  composed  mainly  of  black  basalt  in  horizontal 
layers,  which  assumes  a  great  variety  of  cathedral-like  and 
monumental  forms  on  weathering.  The  architecture  is 
locally  varied  where  granite  and  schist  are  exposed  in  the 
lower  portions  of  the  profound  gulf,  but  throughout  hun- 
dreds of  miles  of  great  escarpments  the  dark  basalt  gives  a 
sombre  and  even  an  oppressive  gloom  to  the  strange  scenery. 
In  its  deepest  portion,  on  the  east  flank  of  the  Blue  Mount- 
ains, the  canyon  is  about  four  thousand  feet  deep  and 
fifteen  miles  broad.  As  in  the  vast  canyon  carved  by  the 
Colorado,  ridges  and  abutments  from  the  main  walls  extend 
from  either  side  far  into  the  profound  depths,  and  fill  the 
depression  so  as  to  make  it  appear  much  narrower  and 
deeper  than  it  is  in  reality. 

The  Columbia  also  flows  in  a  canyon-like  valley  for  much 
of  its  course  after  leaving  the  Rocky  Mountains.  In  its 
wild  passage  through  the  Cascade  Mountains,  it  is  bordered 
by  some  of  the  most  rugged  river  scenery  to  be  found  on 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     27<) 


the  P-icific  coast,  but  nowhere  has  it  formed  a  canyon  com- 
parable with  that  traversed  by  Snake  River. 

The  main  subjects  of  interest  to  admirers  of  bold  scenery  - 
as  well  as  to  the  student  of  topographic  forms  and  of  stream 
development,  presented  by  the  vast  region  drained  by  the 
Columbia,  centre  in  the  relation  of  the  drainage  lines  to  the 
disturbances  which  have  affected  the  rocks.  In  the  Appa- 
lachians, as  we  have  seen,  many  of  the  rivers  flow  across 
folded  rocks  and  have  cut  water-gaps  through  the  ridges; 
in  the  region  drained  by  the  Columbia  the  streams  fre- 
quently cross  rocky  ridges  formed  by  the  upraised  edges  of 
tilted  blocks  of  the  earth's  crust,  and  also  give  origin  to 
water-gaps.  In  several  instances  sharp-crested  walls  of  rock 
from  a  few  hundred  to  two  or  three  thousand  feet  high, 
have  been  upraised  directly  athwart  the  course  of  the 
Columbia  or  of  its  branches,  but  the  rivers  have  not  been 
turned  aside.  As  the  blocks  were  tilted  and  their  edges 
slowly  elevated,  the  rivers  deepened  their  channels  as  fast 
as  the  land  rose  and  thus  maintained  their  right  of  way.  In 
other  instances,  the  waters  were  held  in  check  for  a  time  by 
the  rising  land,  and  caused  lakes  to  form,  but  the  barriers 
were  slowly  cut  across  by  the  out-flowing  streams,  and 
again,  what  may  be  termed  gateways  were  opened  through 
the  ridges.  The  thickness  of  ancient  lake  scdhnents  over 
broad  areas  in  the  region  under  discussion  shows  that  earth- 
movements,  similar  to  those  which  influenced  the  character  of 
the  present  topography,  have  been  long  in  progress  and  have  ^ 
produced  profound  geological  as  well  asgeographical changes. 

The  tilting  of  blocks  of  the  earth's  crust  in  the  region  ~ 
drained  by  the  Columbia  has  not  only  produced  ridges  of 


S 


280 


RIVERS  OF  NORTH  AMERICA 


the  nature  just  referred  to,  but  in  certain  instances  the 
surface  has  been  depressed,  thus  lessening  the  grade  of  the 
streams  and  causing  them  to  deposit  their  loads  and  ag- 
grade their  valleys.  Broad  areas  have  for  this  reason  been 
transformed  into  nearly  level  alluvial  plains.  ' 

In  the  instructive  Columbian  region,  and  especially  in 
that  portion  of  central  Washington  known  as  the  Big  Bend 
country,  where  the  climate  is  now  arid  and  the  rate  of  gen- 
eral waste  from  the  surface  due  to  atmospheric  agencies 
small,  lines  of  fracture  and  of  moderate  faulting  have  deter- 
mined the  direction  of  the  stream  courses.  The  streams 
flow  along  lines  of  fracture  and  in  some  instances  have  ex- 
cavated canyons  with  one  wall  higher  than  the  other.  This 
is  the  only  region  in  North  America,  so  far  as  has  been 
recognised,  where  the  relation  of  streams  to  fractures  in  the 
earth's  crust  favours  the  once  prevalent  idea  that  valleys 
are  due  to  breaks  in  the  rocks,  instead  of  resulting  from  the 
wearing  action  of  streams.  Even  these  minor  examples, 
however,  of  the  influence  of  fractures  on  drainage  fail  ta 
support  the  hypothesis  referred  to,  since  the  breaks  simply 
gave  direction  to  the  streams  which  subsequently  excavated 
the  valleys,  instead  of  directly  producing  the  depressions. 

Another  feature  of  especial  interest  in  the  land  of  the 
"  Oregon,"  illustrating  the  influence  of  climate  on  the  lives 
of  streams,  is  furnished  by  the  Grand  Coulee,  a  deep,  steep- 
sided  canyon  which  cuts  across  the  plateau  partially  enclosed 
by  the  Big  Bend  of  the  Columbia.  The  Columbia  once 
flowed  through  this  great  trench,  having  been  turned  from 
its  present  channel  by  the  advance  of  a  glacier  from  the 
mountains  to  the  north.      This  dam  of  ice  held  the  river  in 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     2%\ 


ices  the 
le  of  the 
and  ag- 
;on  been 

cially  in 
lig  Bend 
:  of  gen- 
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70.  deter- 
streams 
have  ex- 
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simply 
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ssions. 

of  the 
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enclosed 
)ia  once 
ed  from 
rom  the 
river  in 


check  and  caused  it  to  rise  rind  form  a  long,  narrow  lake, 
the  outlet  of  which  was  through  the  previously  eroded 
canyon  now  known  as  the  Grand  Coulee. 

The  Columbia  flows  directly  through  the  Cascade  Mount- 
ains nearly  at  right  angles  to  their  trend,  in  a  wild  and  ex- 
ceedingly picturesque  water-gap,  the  greatest  of  its  class  on 
the  continent.  The  complete  history  of  this  most  impres- 
sive topographical  feature  has  not  been  made  out,  but  the 
facts  in  hand  suggest  that  the  mountains,  like  the  narrow, 
sharp-crested  ridges  to  the  eastward,  are  due  to  the  upraising 
of  a  block  or  a  series  of  blocks  of  the  earth's  crust,  along  a 
line  or  belt  of  faulting,  and  that  the  river  deepened  its 
channel  as  fast  as  the  rocks  rose.  Possibly  the  elevation  of 
the  land  was  not  uniform,  but  progressed  by  stages,  and  that 
when  most  rapid,  the  river,  unable  to  maintain  its  grade,  was 
ponded  and  lakes  formed.  This  explanation  of  the  origin  of 
the  Dalles  of  the  Columbia,  and  other  gorges  both  above  and 
below,  must  not  be  accepted  too  hastily,  however,  as  the 
possibility  of  cross-fractures  having  given  direction  to  the 
river  and  assisted  it  in  its  task  has  not  been  fully  considered. 

Where  the  Columbia  nears  the  ocean  its  waters  lose  their 
energy  and  expand  into  an  estuary,  in  which  the  tides  rise 
and  fall  for  a  distance  of  about  one  hundred  and  forty  miles 
from  the  ocean.'     From  what  has  been  said  concerning  the 

'  Rev.  Earl  M.  vVilbur  of  Portland,  Oregon,  has  informed  the  writer  by  let- 
ter, on  the  authority  of  government  engineers,  "  that  the  lide  is  felt  in  the 
Columbia  as  far  as  the  Lower  Cascades,  which  is,  I  believe,  about  140  miles 
from  the  mouth  of  the  river ;  the  extreme  range  there  being  aliout  six  inches. 

"  In  the  Willamette,  the  Columbia's  largest  affluent,  the  tide  is  felt  as  far  as 
Oregon  City,  where  there  are  falls  ;  about  115  miles  from  the  ocean. 

"  The  extreme  range  noted  at  Portland,  about  icx)  miles  from  the  ocean,  is 
3.2  feet." 


'k 

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282 


JirVERS  OF  NORTH  AMERICA 


drowning  of  stream-cut  valleys  both  on  the  Atlantic  and 
Pacific  coasts,  it  will  be  readily  seen  that  a  modern  subsi- 
dence has  recently  affected  a  great  extent  of  the  coastal 
region  of  the  North-west,  including  south-eastern  Alaska, 
and  has  allowed  the  sea  to  encroach  on  the  land  and  trans- 
form the  lower  courses  of  many  valleys  into  tideways. 

The  extremely  interesting  problems  presented  by  the 
region  drained  by  the  Columbia,  and  the  magnificence  and 
novelty  of  the  scenery  existing  there,  tempt  me  to  detain 
the  reader  and  consider  more  fully  the  origin  of  the  mas- 
sive cliffs  and  of  the  terraces  and  landslides  on  their  faces. 
There  are  yet  other  connections  between  the  elements  of 
scene';-  and  the  work  of  streams,  and  a  wonderful  story  of 
the  time  when  the  land  was  again  and  again  inundated  by 
floods  of  molten  lava,  but  as  the  object  of  this  fireside  recon- 
noissance  is  simply  to  indicate  some  of  the  more  instructive 
features  of  the  land  which  the  study  of  streams  assists  in 
interpreting,  we  must  hasten  on.     Our  journey  is  northward. 

Rivers  of  the  Far  North-  West. — Fraser  River,  fed  by  tens 
of  thousands  of  twig-like  branches  on  the  western  slope  of 
the  Cordilleran  Moimtain  system,  furnishes  much  informa- 
tion in  reference  to  the  manner  in  which  a  broad,  high 
region  becomes  dissected  by  the  streams  flowing  from  it. 
Many  of  the  bran^^hes  of  this  splendid  river  have  their 
sources  in  fine  glaciers,  high  up  among  the  glorious  peaks 
of  the  Selkirks  and  neighbouring  ranges,  flow  through  wild, 
steep-sided  gorges  and  valleys  and  unite  to  form  a  trunk 
stream  which  has  sunken  three  or  four  thousand  feet  into 
the  rocks.  In  following  the  steep  bank  of  the  Fraser,  while 
making   the   transcontinental  journey    over   the  Canadian 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     283 


antic  and 
srn  subsi- 
e  coastal 
1  Alaska, 
nd  trans- 
ays. 

1   by  the 
ence  and 
:o  detain 
the  mas- 
eir  faces, 
ments  of 
1  story  of 
dated  by 
de  recon- 
structive 
issists  in 
rthward. 
I  by  tens 
slope  of 
nforma- 
id,   high 
from  it. 
ve  their 
is  peaks 
jh  wild, 
a  trunk 
jet  into 
r,  while 
anadian 


Pacific,  one  sees  on  every  hand  evidences  of  the  work  of 
streams.  The  topography  is  yet  young,  although  deeply 
and  boldly  cut,  but  the  valleys  are  narrow,  barely  wide 
enough  to  give  the  rushing,  foaming  waters  a  passageway. 

Throughout  much  of  the  trunk  portion  of  the  Fraser 
drainage-tree,  the  grade  is  sufficiently  steep  to  insure  a 
rapid  current.  The  debris  brought  from  glaciers,  and  fed 
by  tributary  rills  and  creeks,  supplies  the  swiftly  running 
waters  with  an  abundance  of  tools  with  which  to  deepen 
their  channels.  Many  conditions  favour  rapid  work,  and  it 
is  not  surprising  that  the  swift,  debris-charged  river  has 
literally  sawed  a  great  mountain  system  into  blocks,  and  is 
progressing  rapidly  vi^^^h  the  task  of  removing  the  masses 
still  remaining  between  its  branches.  The  walls  of  its  main 
canyon,  although  less  precipitous  than  the  bordering  cliffs 
of  the  Colorado  or  the  Snake,  are  wonderfully  varied  and 
picturesque.  When  seen  from  below  they  appear  like  deeply 
sculptured,  forest-clothed  mountain  ranges.  The  river  is 
yet  young,  but  has  accomplished  a  herculean  task,  and  is 
still  working  with  the  energy  of  youth.  As  in  the  case  of 
Snake  River,  the  Fraser  was  interrupted  in  its  work  of  cor- 
rasion  during  the  Glacial  epoch  and  its  canyon  deeply  filled ; 
more  recent  corrasion  has  removed  much  of  the  alluvium, 
however,  leaving  well-marked  terraces,  as  is  illustrated  on 
Plate  VIII.  Its  canyon,  although  three  or  four  thousand 
feet  deep,  has  not  yet  reached  the  limit  to  which  down- 
cutting  is  possible.  Vertical  corrasion  is  still  in  excess  of 
lateral  wear  and  weathering,  and  the  great  trench  is  V- 
shaped  in  cross-section  instead  of  being  broadly  U-shaped, 
as  will  be  the  case  in  its  mature  life.     Like  the  streams  of 


^^^^^ 


[ft  I 

111'  1 


•       .:»]'■ 


284 


RIVERS  OF  NORTJJ  AMERICA 


the  Sierra  Nevadas,  its  energy  is  not  all  consumed  in  trans- 
porting the  debris  delivered  to  it,  and  for  a  large  part  of 
the  year  it  rushes  along  as  a  foaming,  roaring  torrent  carry- 
ing its  load  easily  until  it  enters  the  coastal  region,  where 
a  recent  depression  of  the  land  causes  a  decrease  in  grade 
and  a  consequent  loss  of  velocity.  The  river  is  shorter 
than  formerly,  for  the  reason  that  its  trunk  near  the  sea 
has  been  transformed  into  an  estuary.  The  drainage-tree 
has  been  betrunked  by  subsidence  and  drowning. 

Glacier-Born  Rivers. — North  of  the  Fraser,  and  similar  to 
it  in  the  chief  points  of  their  histories,  are  the  Stickine, 
Taku,  Alsec,  and  other  rivers  which  have  their  sources  to 
the  east  and  north  of  the  mountains  near  the  coast  and  flow 
through  rugged  and  as  yet  but  little-known  lands  to  the  sea. 
Probably  all  of  the  region  drained  by  these  rivers  was  ice- 
covered  at  a  comparatively  recent  date,  and  thousands  of 
glaciers  still  remain.  Many  are  the  lessons  illustrated  by  the 
rugged  landscapes  of  British  Columbia  and  Alaska  of  the 
manner  in  which  streams  and  glaciers  modify  topography, 
,  td  the  way  that  a  subsidence  of  a  deeply  dissected  land 
leads  to  the  production  of  a  ragged  coast-line  fringed  with 
islands.  This  region  includes  the  highest  mountain  and  the 
largest  glaciers  in  North  America.  The  chief  lesson  that 
invites  the  geographer  amid  the  ice-covered  mountains  near 
the  coast  is  the  influence  of  climate  and  of  topography  on 
the  birth,  growth,  and  decline  of  glaciers.  This  theme  has 
been  considered  in  a  preceding  volume.' 

The  streams,  many  of  them  veritable  rivers,  flowing  be- 
neath  the   glaciers,    make    highly    interesting   deposits   of 

'  I.  C.  Russell,  Glaciers  of  North  America.     Ginn  &  Co.,  Boston,  1897. 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     285 


ed  in  trans- 
rge  part  of 
rrent  carry- 
jion,  where 
56  in  grade 
'  is  shorter 
sar  the  sea 
ainage-tree 

i  similar  to 

e  Stickine, 

sources  to 

st  and  flow 

to  the  sea. 

irs  was  ice- 

3usands  of 

ited  by  the 

ka  of  the 

3ography, 

ected  land 

nged  with 

n  and  the 

sson  that 

tains  near 

raphy  on 

heme  has 

jwing  be- 
posits   of 

jn,  1897. 


gravel  in  the  tunnels  they  occupy,  and  form  alluvial  cones 
and  broad  sand-plains  after  escaping  from  the  ice.  The 
study  of  these  peculiar  accumulations,  still  in  process  of 
formation,  furnishes  an  explanation  of  many  riddles  in 
formerly  glaciated  lands. 

To  the  north  of  the  narrow  coastal  portion  of  southern 
Alaska,  where  the  surface  waters  are  discharged  directly  to 
the  Pacific,  lies  the  great  region  drained  principally  by  the 
Yukon,  which  forms  the  Bering  drainage  slope.  The  hydro- 
graphic  basin  of  the  Yukon  embraces  about  440,000  square 
miicb,  and  the  volume  of  the  river,  although  as  yet  un- 
measured, is  comparable  with  that  of  the  Mississippi. 

The  Yukon  presents  many  of  the  characteristics  of  the 
rivers  of  more  southern  latitudes,  and  also  possesses  certain 
features  peculiar  to  the  streams  of  northern  countries. 
Flowing,  as  it  does  in  its  upper  course,  from  south  to  north, 
the  wave  of  sunshine  and  warmth  that  sweeps  from  the 
equatorial  to  polar  regions  each  recurring  springtime 
reaches  the  lands  drained  by  its  head-waters  and  loosens 
the  icy  grasp  of  winter,  while  its  lower  portion  is  still  ice 
bound.  The  melting  of  the  snow  and  ice  and  the  spring 
rains  at  the  south  cause  the  streams  to  rise  in  floods,  which 
advance  laden  with  floating  ice  upon  the  still  frozen 
country  to  the  northward.  Ice-dams  are  formed,  and  the 
streams  expand  and  inundate  the  forest-covered  valley 
bottoms.  The  rising  waters  finally  break  the  ice-dams  and 
rush  on  down  the  valleys  carrying  destruction  in  their  paths. 
Trees  are  uprooted,  or  cut  off  by  the  floating  ice  as  with  a 
scythe.  Vast  quantities  of  earth  and  stones,  enclosed  in 
the  ice  that  formed  in  shallow  water,  are  borne  along  and 


111  11 


''1 

III  A 


I' 


c 
c 

r 

¥ 


286 


Avr'/i'/'.s-  (>/■  X(U"/7/  A  mi: A' /C A 


deposited  in  p.irt  over  the  flood-pliiiiis  of  the  streams  when 
the  ice  melts.  The  enerj^y  with  which  the  Yukon  modifies 
its  batiks,  on  account  especial  >•  of  the  ice-laden  floods,  is 
unrivalled  by  any  more  southern  river. 

Another  important  variation  in  what  may  be  considered 
as  the  normal  action  of  streams  arises,  in  the  Alaskan 
region,  from  the  constantly  frozen  condition  of  the  soil. 
Throu};hout  nearly  the  whole  of  the  area  drained  by  the 
Yukon  the  soil  in  the  low  lands  is  continually  frozen.  The 
winters  arc  lonj;  and  severe,  the  summers  short  and  hot. 
The  soil  at  a  depth  of  a  few  inches  beneath  the  usual  cover- 
in^i;  of  moss,  shrubs.  ai»d  trees  is  perennially  frozen.  The 
thickness  of  the  frozen  subsoil  is  not  known,  but  excavations 
twenty-five  feet  in  depth  have  failed  to  penetrate  it.  Ice- 
cliffs  along  the  Kowak  River,  in  North-western  Alaska,  re- 
veal a  thickness  of  fully  two  hundred  feet  of  dirt-stained  ice 
beneath  a  thin  layer  of  black  mucky  soil  on  which  grasses 
and  other  vegetation  thrive.  From  these  and  other  observ- 
ations, and  especially  the  records  of  certain  borings  made 
in  a  similar  region  in  Siberia,  it  is  safe  to  assume  that  the 
average  thickkcss  of  the  frozen  layer  in  Alaska  is  probably 
in  excess  of  one  hundred  feet  and  possibly  two  or  three 
hundred  feet  or  more  in  depth.  These  conditions  have  an 
important  bearing  on  the  work  of  streams.  Frost  renders 
otherwise  loose  and  inadherent  material  as  firm  as  solid 
rock.  The  action  of  flowing  waters  on  the  land  is  thus 
checked,  and  stream  development  as  well  as  rock  disintegra- 
tion and  decay  and  general  surface  erosion  greatly  retarded. 

The  climatic  conditions  in  the  region  under  discussion  are 
such  that  the  ground  almost  everywhere  is  covered  with  a 


so^fK  c:/aractI':a'ist/cs  of  amekicas'  riveks    287 


■earns  when 
on  modifies 
n  floods,  is 

considered 

le   Ahiskan 

3f  the  soil. 

led   by   the 

>zen.     The 

t  and  hot. 

sual  cover- 

)zen.     The 

:xca  vat  ions 

te  it.     Ice- 

Ahiska,  rc- 

[stained  ice 

I  grasses 

\x  observ- 

in^s  made 

that  the 

probably 

or  tliree 

IS  have  an 

st  renders 

as  sohd 

d   is  thus 

isintegra- 

retarded. 

ssion  are 

xl  with  a 


dense  growth  of  mosses  and  Hchens,  which  make  a  Hving 
mat  through  which  the  surface  waters  percolate  as  through 
a  layer  of  sponges,  and  are  filtered  of  all  matter  in  suspen- 
sion. Thus,  again,  the  work  of  the  streams  is  delayed,  for 
the  reason  that  sand  and  silt,  which  ordmarily  constitute  the 
principal  tools  with  which  flowing  waters  abr  1  the  rocks, 
are  removed.  This  process  of  filtering  the  .va:  r  is  illus- 
trated by  the  contrasts  in  the  character  of  ti  ■  t  .butaries  of 
the  Yukon  which  come  to  it  from  the  north  and  from  the 
south.  Every  stream,  so  far  as  is  known,  which  joins  the 
great  river  along  its  right  bank  is  clear,  although  usually 
amber-coloured  on  account  of  the  organic  material  contained 
in  solution ;  while  the  tributaries  entering  from  the  left,  or, 
in  general,  the  southern  bank,  are  mostly  turbid  and  heavily 
loaded  with  sediment,  for  the  reason  that  they  have  their 
sources  in  glaciers.  White  River,  one  of  the  principal 
tributaries  of  the  Yukon  from  the  south,  is  charged  with 
material  in  suspension  not  only  because  it  is  fed  by  melting 
glaciers,  but  for  the  rea.son  that  it  flows  through  a  region 
that  is  covered  with  fine  volcanic  dust,  some  of  which  is 
washed  into  the  stream  by  every  rain.  The  accidents  to 
streams,  as  they  have  been  termed,  due  to  glacial  and  to 
volcanic  agencies  here  find  abundant  illustration. 

The  valley  of  the  Yukon  and  of  several  of  its  important 
tributaries,  particularly  to  the  east  of  the  Alaskan  boundary, 
are  marked  by  conspicuous  terraces.  A  part  of  these  are 
lake  terraces,  formed  at  a  time  when  the  waters  were  held  in 
check  by  a  lava  dam,  but  other  and  equally  conspicuous 
terraces  were  formed  by  the  streams,  and  record  changes  in 
the  altitude  of  the  land,  or  the  ovcrloaJing  of  the  waters 


c 
c 


288 


RIVERS  OF  NORTH  AMERICA 


with  detritus  during  a  time  when  glaciers  near  their  sources 
were  much  more  abundant  and  of  far  larger  size  than  now. 
The  head-branches  of  the  Yukon  drainage -tree  rise  in  a 
country  which  was  formerly  covered  with  a  continuous  ice- 
sheet,  but  in  its  middle  and  lower  courses  evidence  of  for- 
mer ice  occupation  is  wanting.  Marked  differences  in  the 
scenery  beheld  in  journeying  from  one  of  these  regions  to 
the  other  have  been  noted  by  several  travellers. 

Bering  Sea,  into  which  the  Yukon  empties,  is  shallow, 
at  least  in  the  portions  bordering  Alaska,  and  is  without 
strong  currents  or  high  tides.  The  great  river  on  entering 
the  sea  drops  its  heavy  burden  of  silt,  and  has  built  up  a 
delta  comparable  in  extent  with  that  of  the  Mississippi. 
The  river  divides  into  many  branches,  or  sends  off  several 
distributaries  in  the  delta  portion  of  its  course.  The  first 
of  these  diverging  channels  leaves  the  main  river  about  a 
hundred  miles  from  its  mouth.  The  low,  swampy  area 
built  by  the  stream  is  treeless,  but  clothed  in  summer  with 
a  dense  growth  of  mosses,  lichens,  grasses,  rushes,  and  a 
great  variety  of  less  conspicuous  flowering  plants.  This 
luxuriant  garden  of  brilliant  flowers  and  luscious  green 
fronds  and  leaves  is  but  a  veneer  of  verdure  concealing  a 
frozen  morass.  This  is  a  portion  of  the  vast  treeless  tract 
of  perennially  frozen  ground  known  as  the  tundra,  which 
fringes  the  shores  of  Bering  Sea  and  the  Arctic  Ocean. 
The  several  distributaries  of  the  river  flow  through  this  new- 
n  Je  land  in  meandering  courses,  and  enter  the  sea  at 
various  localities,  over  a  breadth  of  seventy  miles  of  coast. 

With  the  exception  of  the  delta  portion  of  the  Yukon,  its 
banks   aie   fringed    with   spruce    trees,    cottonwoods,    and 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     289 


ir  sources 
han  now. 
rise  in  a 
uous  ice- 
ce  of  for- 
es in  the 
egions  to 

shallow, 
i  without 
entering 
)uilt  up  a 
ississippi. 
iff  several 
The  first 
r  about  a 
npy  area 
er  with 
s,  and  a 
s.     This 
IS  green 
ealing  a 
ss  tract 
,  which 
Ocean. 
lis  new- 
sea  at 
coast, 
kon,  its 
s,    and 


willows.  The  annual  floods  and  ice-gorges  cause  large 
numbers  of  trees  to  be  swept  into  the  river,  and  the  swift 
current  at  numerous  localities  cuts  away  the  banks  in  such 
a  way  as  to  undermine  the  trees  growing  on  them  and  cause 
them  to  fall  into  the  waters  with  roots  and  branches  at- 
tached. Great  quantities  of  drift-wood  are  thus  contributed 
to  the  river,  as  has  already  been  described.  The  conserva- 
tive influence  both  of  growing  trees  and  of  stranded  drift- 
wood on  the  banks  of  the  river  are  well  marked  and 
important,  as  are  also  the  destructive  tendencies  of  the 
same  agencies.  The  trees  on  being  uprooted  tear  away  the 
banks,  and  on  being  stranded  frequently  deflect  the  current 
so  as  to  cause  it  to  cut  away  neighbouring  shores  and  in- 
crease the  number  and  extent  of  the  windings  of  the  river. 
Arctic  Rivers. — Of  the  streams  flowing  down  the  Arctic 
drainage  slope  but  little  can  be  said,  for  the  reason  that 
no  traveller  especially  interested  in  the  study  of  modern 
geography  has  visited  that  region.  The  Mackenzie  prob- 
ably illustrates  the  characteristics  of  a  northward-flowing 
Arctic  river  even  bettor  than  the  Yukon.  Much  of  the 
region  it  traverses  is  forested,  and  vast  floods  occur  each 
spring  when  the  thick  ice  of  winter  breaks  up  and  is  swept 
northward.  The  sudden  changes  that  occur  at  the  turn  of 
the  annual  tide  of  temperature  must  be  even  grander  than 
along  the  Yukon,  but  in  this  connection  but  liille  informa- 
tion is  available.  On  entering  the  Arctic  Ocean  when  the 
tides  are  low  and  currents  produced  by  the  winds  mostly 
lacking,  owing  to  the  fact  that  the  sea  is  covered  with  ice- 
floes throughout  the  year,  the  river  deposits  it.;  sediment 
and  is  engaged  in  building  a  large  delta.     The  three  gieat 


)mi 


290 


RIVERS  OF  NORTH  AMERICA 


:iii 


r 


III 


delta-making  rivers  of  North  America  are  the  Mississippi, 
Yukon,  and  Mackenzie. 

Rivers  of  the  "  Great  Lone  Land.'' — On  the  Hudson  Bay- 
drainage  slope  there  are  tens  of  thousands  of  lakes,  which, 
for  the  most  part,  occupy  basins  due  in  one  way  or  another 
to  the  former  occupation  of  the  land  by  glacial  ice.  There 
are  also  many  rivers,  but  the  way  in  which  they  illustrate 
the  principles  of  stream  development  has  received  but 
slight  attention.  The  reports  of  explorers,  especially  those 
connected  with  the  Geological  and  Natural  History  Survey 
of  Canada,  show  that  the  drainage  is  immature.  The 
streams  have  not  cut  down  their  channels  so  as  to  furnish 
direct  and  ready  avenues  of  discharge  for  the  surface  waters. 
This  is  demonstrated  especially  by  the  countless  lakes.  The 
streams  are  not  only  young,  having  come  into  existence  or 
having  been  rejuvenated  since  the  last  retreat  of  the  glaciers, 
but  have  developed  slowly  on  account  of  adverse  circum- 
stances. Among  the  conditions  that  have  retarded  stream 
development  may  be  noted  the  general  low  altitude  of  the 
land,  and,  consequently,  gentle  gradients  of  the  stream 
channels  and  lack  of  energy  in  the  flowing  waters.  The 
winter  climate  is  severe,  and  the  streams  either  ice-covered 
or  frozen  to  their  bottoms  for  several  months  each  year. 
Snow  protects  the  ground  in  winter.  The  subsoil,  as  in  the 
Yukon  basin,  remains  solidly  frozen  in  many  places  even 
during  the  warm  season.  Erosion  and  the  transportation 
of  debris  by  the  streams  is  thus  limited  to  one  half,  or  even 
less,  of  the  year.  Forests  with  iindergrowths  of  mosses, 
lichens,  and  other  plants  shield  the  soil  in  summer  from  the 
beating  of  rain,  and  filter  the  percolating  surface  waters, 


ississippi, 

dson  Bay- 
is,  which, 
>r  another 
I.     There 
illustrate 
2ived    but 
ally  those 
ry  Survey- 
ire.      The 
to  furnish 
ce  waters, 
kes.     The 
:istence  or 
e  glaciers, 
circum- 
d  stream 
de  of  the 
e   stream 
rs.     The 
e-covered 
ach  year, 
as  in  the 
ices  even 
portation 
,  or  even 
mosses, 
I  from  the 
waters, 


SOAfE    CHARACTERISTICS  OF  AMERICAN  RIVERS     29I 

thus  robbing  them  of  the  means  of  abrading  the  rocks  over 
which  they  flow.  There  are  no  glaciers  to  supply  the 
streams  with  sediment.  The  vegetation  retards  the  gather- 
ing of  the  waters  into  rills,  and  equalises  the  flow  of  the 
streams  in  such  a  manner  that  the  floods  caused  by  melting 
snow  have  their  energy  diminished.  The  river  banks  are 
clothed  with  trees  and  shrubs,  especially  willows  and  alders, 
and  their  roots  bind  the  soil  and  increase  its  ability  to  resist 
the  attacks  of  the  flowing  waters.  Drift  timber  lodged 
against  the  sides  of  the  streams,  especially  fallen  trees  which 
still  retain  a  hold  on  the  land,  also  protect  the  banks.  For 
these  and  still  other  reasons,  tlie  work  of  the  streams  has 
progressed  slowly.  They  illustrate  retarded  stream  develop, 
ment,  or  a  long-continued  youthful  stage.  In  this  respect 
thcv  afford  a  marked  contrast  to  the  Colorado,  where  the 
opportunities  for  development  have  been  unusually  great. 
There  is  still  another  reason  for  the  slow  development  of 
the  streams  flowing  to  Hudson  Bay,  although  its  full  signifi- 
cance has  not  been  determined.  That  is,  the  region  toward 
which  they  flow  is  believed  to  be  a  rising  area.  The  upward 
movement  of  the  land  is  slow,  although  the  rate  is  not 
known.  An  elevation  of  a  very  few  inches  a  century  would 
have  a  decided  effect  on  the  flow  of  the  streams  in  a  region 
of  such  mild  relief. 

A  glance  at  a  maji  of  North  America  suggests  that  a  large 
number  of  islands  into  which  the  land  is  broken  on  the 
north-eastern  border  of  the  continent  is  due  to  a  recent  sub- 
sidence. There  is  geological  evidence  that  over  a  vast  area 
at  the  north,  the  land  was  depressed  during  the  Glacial 
epoch  and  has  since  been  slowly  rising,  but  has  not  regained 


292 


RIVERS  OF  NORTH  AMERICA 


the  birth  of  the 


-sheets 


the  position  it  held  previous 
v'hich  once  covered  it.  This  re-elevation  is  thought  to  be 
still  in  progress,  and  should  this  conclusion  be  maintained 
it  will  furnish  an  additional  reason  for  the  present  immature 
condition  of  the  northward-flowing  streams  just  referred  to. 
Rivers  Flowing  to  Fresh-Water  Seas. — The  St.  Lawrence 
drainage  slope,  with  its  great  lakes  and  magnificent  rivers, 
affords  numerous  features  of  interest  to  the  geographer  be- 
sides its  beautiful  scenery.  Soundings  made  in  the  Gulf  of 
St.  Lawrence,  and  even  well  to  the  eastward  of  the  most 
easterly  cape  of  Nova  Scotia,  have  revealed  the  fact  that 
the  submerged  channel  of  the  St.  Lawrence  River  may  be 
traced  on  the  floor  of  the  ocean  as  far  as  the  submarine  es- 
carpment marking  the  true  continental  border.  From  the 
eastern  extremity  of  this  submerged  channel  through  the 
Gulf  of  St.  Lawrence  and  up  the  narrowing  estuary  to  near 
Montreal,  where  the  river  at  present  meets  tide-water,  is 
more  than  a  thousand  miles.  The  Saguenay  River,  bordered 
by  towering  walls,  occupies  a  canyon  excavated  by  a  branch 
of  the  Greater  St.  Lawrence.  The  same  conditions  are 
recorded  in  a  less  marked  way  by  the  Ottawa  and  other 
branches  of  the  present  river.  We  have  here  the  most 
remarkable  example  of  a  drowned  river-system  that  is 
known.  The  marginal  portion  of  the  continent  with  broad 
valley  near  the  sea,  leading  inland  to  deep  canyons,  has 
been  depressed  in  recent  times  so  as  to  allow  the  sea  to  en- 
croach on  the  land.  The  valleys  have  become  gulfs,  bays, 
and  estuaries,  and  the  canyons  narrow  tideways;  highlands, 
that  separated  the  former  river  valleys,  when  not  completely 
submerged  have  been  transformed   into  capes    and    head- 


■  't-:-. 


;e-sheets 
ht  to  be 
intained 
nmature 
;rred  to. 
,awrence 
it  rivers, 
pher  be- 
;  Gulf  of 
;he  most 
fact  that 
r  may  be 
larine  es- 
^rom  the 
)ugh  the 
/  to  near 
water,  is 
ordered 
la  branch 
ions  are 
d  other 
lie  most 
that   is 
h  broad 
ns,   has 
a  to  en- 
s,  bays, 
hlands, 
pletely 
ll    head- 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     293 

lands,  and  in  part  surrounded  by  the  sea  so  as  to  form 
islands. 

The  influence  of  the  geographical  history  of  the  St.  Law- 
rence on  the  course  of  human  events  is  even  more  strongly 
marked  than  in  the  case  of  similar  changes  along  the  Atlantic 
border  to  the  southward.  The  estuary  of  the  St.  Lawrence 
furnished  an  easy  passageway,  reaching  far  inland,  for  early 
explorers,  and  its  connection,  by  means  of  the  unsubmerged 
portion  of  the  ancient  river,  with  the  Great  Lakes  tempted 
the  Jesuit  missionaries  to  make  bold  canoe  journeys  into 
the  very  heart  of  the  continent.  This  same  route  led  to  the 
establishment  of  missions  and  the  planting  of  white  settle- 
ments and  trading  stations  on  the  shore  of  the  Great  Lakes 
and  in  the  Mississippi  valley  before  the  passes  in  the  Ap- 
palachians to  the  southward  became  known.  In  later  years, 
a  series  of  canals  to  facilitate  navigation  between  the  St. 
Lawrence  estuary  and  the  Great  Lakes  stimulated  industry 
by  bringing  tens  of  thousands  of  square  miles  of  forest 
and  of  rich  agricultural  land  into  communication  with  the 
markets  of  Europe.  Far-reaching  plans  for  establishing 
deep  waterways  along  this  general  course  of  early  canoe 
navigation  are  now  being  matured,  and  the  influence  of 
geographical  conditions  favourable  to  commerce  will  be  felt 
still  more  potently  in  the  future  than  they  have  been  in  the 
past.' 

To  the  south  of  the  St.  Lawrence  estuary  lies  the  charm- 
ing valley  of  Lake  Champlain,  which  was  excavated  by  a 
stream  tributary  to  the  Greater  St.  Lawrence  when  the  land 

~  'I.  C.  Russell,  "Geography  of  the  Laurentian  Rnsin,"  in  Bulletin  0/  th* 
American  Geof^raphical  Society,  vol.  xxx.,  pp.  226-254,  1898. 


It'll 


294 


RIVERS  OF  NORTH  AMERICA 


Stood  higher  than  now.  After  acquiring  about  its  present 
form,  the  Champlain  valley  was  depressed  and  became  an 
arm  of  the  sea,  which  was  inhabited  by  marine  mollusks 
and  frequented  by  whales.  A  tideway  reaching  southward 
connected  with  the  submerged  Hudson  River  valley,  making 
New  England  an  island.  A  partial  re-elevation  of  the  land 
caused  the  former  gulf  to  be  separated  from  the  ocean,  so 
as  to  form  a  saline  lake.  The  rains  and  feeding  streams 
furnished  a  supply  of  fresh  water  in  excess  of  the  amount 
lost  by  evaporation,  and  the  salt  waters  were  flooded  out 
and  the  present  stage  in  the  history  of  the  valley  initiated. 
This  marvellous  transformation  of  a  broad  and  well-de- 
veloped river  valley  to  an  arm  of  the  sea,  to  a  saline  lake, 
and  then  to  a  fresh  lake,  in  which  the  blue  Adirondack  Hills 
and  the  equally  picturesque  mountains  of  Vermont  are  re- 
flected, is  one  of  the  most  instructive  pages  in  the  later 
geographical  history  of  America. 

The  story  of  the  St.  Lawrence  valley  and  its  tributary 
branches  is  supplemented  by  the  no  less  instructive  history 
of  the  basins  of  the  Great  Lakes,  some  account  of  which 
has  been  given  in  a  companion  to  the  present  volume.' 

The  student  of  river  development  and  of  the  changes  made 
by  streams  in  the  topography  of  the  land,  as  he  sails  the 
Great  Lakes  and  visits  the  thriving  cities  on  their  shores, 
sees  records  of  a  time  when  rivers  flowed  through  the  now 
water-filled  basins,  and  for  ages  worked  slowly  at  their  ap- 
pointed task  of  deepening  and  widening  their  valleys.  This 
great  task,  when  far  advanced,  was  more  than  once  inter-- 
rupted  by  the  invasion  of  the  entire  St  Lawrence  region  by 
'  I.  C.  Rubsell,  Lakes  of  North  America.     Ginn  &  Co.,  Boston,  1895. 


)  present 
jcame  an 
mollusks 
)uthward 
,  making 

the  land 
3cean,  so 
;  streams 
i  amount 
oded  out 
initiated. 

well-de- 
line  lake, 
ack  Hills 
nt  are  re- 
the  later 

tributary 
history 
which 
me.' 

jfes  made 
ails  the 
shores, 
the  now 
heir  ap- 
This 
e  inter- 
gion  by 
1895. 


SOME   CHARACTERISTICS  OF  aMEkICAN  RIVERS     295 

glaciers  from  the  north.  Conditions  now  characteristic  of 
central  Greenland  then  prevailed  where  millions  of  homes 
are  now  situated,  and  where  fruitful  farms  have  replaced  the 
desolation  of  ice-fields.  When  the  great  geological  winter 
had  passed,  the  former  stream  channels  were  clogged  with 
debris,  so  as  to  retard  the  waters  and  cause  them  to  choose 
new  courses.  Elevation  and  depression  of  the  land  over 
tens  of  thousands  of  square  miles  still  further  complicated 
the  difificulties  that  the  re-born  streams  had  to  contend  with. 
The  surface  waters  were  held  in  check,  and  formed  vast 
lakes  in  the  partially  obstructed  and  warped  and  deformed 
preglacial  river  valleys. 

When  one  marshals  in  fancy  the  changes  that  the  St. 
Lawrence  drainage  slope  has  passed  through  from  the  time 
when  it  was  more  elevated  than  now  and  supplied  a  well- 
developed  river-system, — that  is,  a  river  with  many  branches, 
which  had  cut  down  its  channel  nearly  to  sea-level,  so  as  to 
have  a  low  gradient  for  two  thousand  miles  or  more,  and  had 
broadened  its  valley  so  as  to  form  a  wide,  open  plain  which 
extended  far  into  the  many  tributary  valleys, — through 
the  marvellous  changes  incident  to  the  glacial  invasions, 
and  the  partial  submergence  beneath  the  sea,  and  the 
partial  re-elevation  of  the  drowned  portion,  the  damming  of 
the  stream  by  glacial  debris,  and  the  changes  due  to  warping 
of  the  earth's  crust,  the  brief  time  that  civilised  man  has 
been  acquainted  with  the  region  becomes  insignificant.  In 
this  hasty  outline  of  a  million  or  more  years  of  geographical 
history,  although  seemingly  crowded  with  important  events, 
all  of  the  changes  experienced  by  the  St.  Lawrence  drainage 
slope  have  not  been  included.     There  is  evidence  that  the 


296 


RIVERS  OF  NORTH  AMERICA 


St.  Lawrence  basin  has  been  in  communication  with  the 
Mississippi  River  drainage.  In  the  development  of  these 
two  great  river-systems,  there  has  been  a  struggle  for  the 
possession  of  the  land  where  they  approach  each  other 
(analogous  in  some  ways  to  the  wars  of  the  French  and 
English  for  the  possession  of  the  same  territory),  which  will 
be  of  interest  to  the  reader  who  has  followed  the  discussion  of 
the  backward  cutting  of  drainage  lines  in  a  preceding 
chapter.  More  than  this,  there  are  suggestions  that  the 
excavation  of  the  basins  of  the  Great  Lakes  was  due  in  part 
to  streams  flowing  southward  instead  of  eastward,  and  that 
a  change  in  direction  was  caused  by  movements  in  the 
earth's  crust  which  are  still  in  progress.  The  student  of 
geo,  phy  thus  finds  two  chief  lines  of  interest  in  the  region 
under  consideration,  one  dealing  with  the  origin  and  history 
of  the  land  forms,  and  the  other  with  their  bearing  and  in- 
fluence on  the  current  of  human  events. 

Niagara. — The  lakes  that  first  came  into  existence  in  the 
Laurentian  basin  during  the  final  retreat  of  the  glaciers 
were  small  and  numerous.  Many  of  them  were  short-lived, 
and  were  drained  as  the  ice-dam  retaining  them  withdrew 
north-eastward  ;  but  some  of  them  expanded  with  the  retreat 
of  the  ice,  and  became  vast  inland  seas,  larger  than  any  of 
their  present  representatives.  At  a  late  stage  in  the  melt- 
ing of  the  glaciers,  the  basir.s  now  occupied  by  Lakes  Erie 
and  Ontario  were  occupied  by  a  single  great  water-body. 
When  the  ice  withdrew  still  more  and  the  Mohawk  valley  was 
uncovered,  a  lower  outlet  became  available  and  the  waters 
escaped  so  as  to  lower  the  lake  and  cause  it  to  be  divided. 
The  lake  in  the  Erie  basin  overflowed  across  the  dividing 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     297 


with  the 
of  these 
le  for  the 
ich  other 
snch  and 
/hich  will 
:ussion  of 
Dreceding 
that  the 
le  in  part 
and  that 
ts  in  the 
:udent  of 
he  region 
d  history 
|g  and  in- 

ce  in  the 
glaciers 
)rt-lived, 
withdrew 
e  retreat 
n  any  of 
he  melt- 
es  Erie 
r-body, 
lley  was 
waters 
ivided. 
ividing 


land  to  the  Ontario  basin,  and  Niagara  River  was  born. 
An  example  of  the  manner  in  which  a  river  may  originate, 
not  previously  considered  in  this  book,  is  thus  furnished. 
The  many  happy  tourists  who  have  listened  to  the  thunder 
of  mighty  Niagara  and  wandered  along  the  brink  of  the 
gorge  occupied  by  the  waters  in  their  mad  course  below  the 
cataract,  have  many  illustrations  thrust  upon  their  atten- 
tion of  the  manner  in  which  streams  modify  the  land.  The 
cataract  was  once  at  the  margins  of  the  bold  escarpment 
near  Lewiston,  and  has  slowly  receded,  leaving  a  great  gorge 
as  a  record  of  its  work.  Unlike  the  magnificent  canyon  of 
the  Colorado,  or  the  almost  equally  remarkable  example 
through  which  Snake  River  flows,  the  steep-walled  gorge  of 
the  Niagara  has  not  been  worn  out  by  the  flow  of  silt-  and 
sand-laden  waters,  but  as  already  described  in  discussing  the 
migration  of  waterfalls,  illustrates  another  process  by  which 
the  land  may  be  deeply  trc-nchcd.  The  waters  of  Niagara 
come  directly  from  a  great  lake  in  which  they  have  left  all 
of  the  sediment  they  may  once  have  held  in  suspension,  and 
are  clear.  The  deep  tourmaline-green  of  the  j)lunging  cata- 
ract is  never  clouded.  Clear  streams,  as  we  are  aware,  have 
but  little  power  to  deepen  their  channels,  as  all  the  debris 
available  for  transportation  is  soon  removed,  and  the  chem- 
ical action  of  the  waters  in  dissolving  the  rock  over  which 
they  flow  is  so  slow  that  only  in  exceptional  instances  are 
they  able  to  deepen  their  channels  more  rapidly  than  the 
adjacent  surface  is  lowered  by  weathering.  How,  then, 
has  the  canyon  below  the  Falls  of  Niagara  been  excavated  ? 
The  energy  of  Niagara  River  available  for  canyon  cutting 
is  concentrated  at  the  base  of  the  cataract.     The  river  came 


ii 


298 


RIVERS  OF  NORTH  AMERICA 


I 


iM  i 


into  existence  with  a  cataract,  which  was  even  grander  when 
it  first  leaped  from  the  crest  of  the  escarpment  at  Lewiston 
than  it  has  been  since  the  first  white  man  kneeled  in  its 
awful  presence.  Owing  to  the  southward  dip  of  the  rocks, 
the  height  of  the  fall  has  been  continually  decreasing  and 
will  continue  to  decrease  until  it  has  receded  to  the  lake 
from  which  the  river  flows,  or  the  westward  tilting  of  the 
land,  known  to  be  in  progress,  diverts  its  waters.  The  sur- 
face rock  along  Niagara  River  is  a  hard  limestone  about 
eighty  feet  thick ;  beneath  this  there  are  soft  shales,  much 
broken  by  joints  and  easy  of  removal.  The  dash  of  the 
spray,  the  grinding  of  ice-blocks,  and,  to  some  extent,  the 
freezing  of  absorbed  water,  leads  to  the  removal  of  the  shale 
so  as  to  leave  the  limestone  above  projecting.  From  time 
to  time  masses  of  the  limestone  break  off  and  fall  into  the 
pool  below.  When  the  plunging  waters  have  sufficient 
power  to  move  these  blocks,  they  are  dashed  against  the 
cliff  and  act  as  millstones  in  deepening  and  widening  the 
basin  below.  When  the  descending  waters  do  not  have 
sufificient  power  to  sweep  about  the  blocks  of  stone,  as  in 
the  case  of  the  American  Fall,  they  accumulate  and  form  a 
talus  slope  which  protects  the  cliffs,  and  retard  their  seces- 
sion. This  explanation,  first  offered  by  Gilbert,  furnishes 
a  reason  for  the  marked  differences  between  the  American 
and  Canadian  portions  of  the  cataract.  Many  other  inter- 
esting and  instructive  features  of  Niagara  are  described  and 
explained  in  the  monograph  just  referred  to. 

Retrospect. — We  have  made  this  rapid  review  of  the  prin- 
cipal drainage  slopes  of  North  America  for  the  purpose  of 
refreshing  our  memories  concerning  the  more  pronounced 


nder  when 
;  Lewiston 
iled  in  its 
the  rocks, 
casing  and 
)  the  lake 
ng  of  the 

The  sur- 
)ne  about 
les,  much 
sh  of  the 
ctent,  the 
the  shale 
rom  time 
1  into  the 
sufficient 
ainst  the 
:ning  the 

ot  have 
|ne,  as  in 
form  a 
lir  seces- 

urnishes 

Imerican 
inter- 

led  and 

le  prin- 
Ipose  of 
lounced 


SOME   CHARACTERISTICS  OF  AMERICAN  RIVERS     299 

geographical  features  of  the  continent  due  to  erosion. 
Another  aim  has  been  to  suggest  questions  which  the  student 
of  geography  will  find  pleasure  in  answering.  I  fear,  how- 
ever, our  hasty  journey  has  in  some  respects  left  an  errone- 
ous impression  on  the  reader's  mind,  for  the  reason  that  in 
considering  each  drainage  slope  only  its  more  pronounced 
features  have  claimed  attention.  If  each  river  appears 
to  be  principally  and  essentially  different  from  all  other 
streams,  and  the  several  drainage  systems  present  an  infinite 
variety  of  disconnected  facts,  modifications  and  corrections 
of  such  ideas  are  necessary.  A  more  detailed  study  of  the 
behaviour  of  streams  will  show  that  law  and  order  prevail. 
From  the  purling  rill  to  the  majestic  river,  where  at  first, 
perhaps,  endless  variety  appears,  the  flow  of  water  and  the 
changes  it  produces  in  the  relief  of  the  land  are  governed  by 
inflexible  laws.  The  streams  one  and  all  are  engaged  in  a 
definite  and  well-circumscribed  task,  which  leads  to  an 
orderly  succession  of  topographic  forms.  When  all  of  the 
modifying  conditions  are  taken  into  account,  the  successive 
changes  experienced  by  a  given  land  area,  from  the  time  of 
its  upraising  above  the  sea,  to  the  time  when  it  is  worn 
down  nearly  to  sea-level  once  more,  are  seen  to  be  as  much 
in  obedience  to  law  as  the  seasonal  changes  in  a  landscape 
or  the  development  of  an  individual  man  from  childhood 
to  old  age. 

Although  the  origin  of  topographic  forms  and  the  many 
metamorphoses  they  undergo,  claim  the  special  attention  of 
students  of  geography,  the  fact  should  be  borne  in  mind 
that  the  knowledge  thus  gained  is  but  the  basis  of  a  more 
profound  study, — the  relation  of  man  to  nature.     The  in- 


300 


K I  VERS  OF  NORTH  AMERICA 


1 

;^ ; 

u  : 

*• 

r-  , 

r       ' 
1 

1 
! 
• 

y 
y 

f         i 

i 
1 

r 

fluence  of  the  earth's  history  on  human  history,  although  in 
many  instances  not  fully  realised,  is  an  underlying  and  ever- 
present  source  of  interest  and  enjoyment  to  the  geologist 
and  geographer. 

The  brief  review  of  some  of  the  characteristics  of  rivers 
given  in  this  chapter,  it  is  hoped,  will  stimulate  a  desire,  es- 
pecially in  American  students,  to  know  more  of  the  many 
and  varied  charms  of  their  native  land. 


Ithough  in 
y  and  ever- 
e  geologist 


:s  of  rivers 
desire,  es- 
the  many 


CHAPTER  IX 


r/ry?  l/f£  history  of  a  river 

AN  application  of  the  laws  governing  the  behaviour  of 
strea  ms  in  interpreting  the  origin  and  history  of  topo- 
graphic forms  can  be  made  in  almost  any  land  area  on  the 
earth.  In  order  to  group  in  a  single  panorama,  however, 
all  of  the  various  phases  which  a  river  passes  through  from 
its  birth  and  youth  to  its  old  age  and  death,  the  conditions 
presented  by  many  streams  in  various  stages  of  growth  and 
decline  have  to  be  combined,  for  the  reason  that  the  life  of 
a  man  is  too  brief  to  enable  him  to  observe  more  than  a  few 
minor  changes  in  the  history  of  a  single  river.  But  know- 
ing the  laws  which  govern  stream  development,  one  can 
easily  picture  in  his  mind  the  leading  events  in  the  life  of 
a  majestic  river  whose  murmurs  we  may  be  pardoned  for 
fancying  make  audible  the  memoir  of  a  million  years. 

In  order  to  sketch  in  outline  the  life  history  of  an  ideal 
river,  let  the  reader  imagine  that  the  floor  of  the  sea  in 
temperate  latitudes  over  an  area  of  n  hundred  square  miles 
has  been  upraised  so  as  to  form  an  island ;  and  trace  the 
changes  which  will  follow  as  the  rain-water  falls  on  its  sur- 
face, and  gathers  into  rills  which  unite  one  with  another  until 


ffi  ; 


302 


RIVERS  OF  NORTH  AMERICA 


\   jf 


I  \ 

r 


a  series  of  rivers  conducts  the  contributions  from  the  clouds 
down  their  shining  courses  to  the  sea. 

The  surface  of  our  imaginary  island  is  mildly  irregular. 
In  the  central  portion  it  has  an  elevation  of  a  thousand  feet, 
and  slopes  gradually  but  somewhat  irregularly  in  all  direc- 
tions to  the  sea.  In  places  the  waters  gather  in  hollows 
and  form  lakes.  These  constCi^ent  lakes  are  soon  filled,  or 
their  overflowing  waters  cut  notches  in  the  rims  of  their 
basins,  and  they  are  drained.  The  first  stream;-  that  are 
born  of  the  showers,  like  the  children  of  men,  have  their 
courses  >r.arked  out  for  them  in  e-^rly  l;fe,  or,  in  more  prosaic 
language,  are  consequent  streams.  Later  in  life  they  carve 
out  their  own  fortunes  and  influence  their  surroundings. 

In  these  fireside  fancies  we  assume  the  point  of  view 
granted  lie  novelist,  to  whom  time  and  distance  offer  no 
limitation.  A  Scott  or  a  Hawthorne  tells  us  with  confidence 
the  most  secret  thoughts  of  a  prisoner  in  his  cell  a  century 
b*u\)re  they  themselves  v,'erc  br>rn.  We  accept  the  illusion 
so  long  as  the  laws  governing  human  nature  are  not  violated. 
Why  should  a  similar  pri\  ilege  be  denied  the  geographer  ? 
Let  us,  then,  trace  the  changes  that  our  island  will  undergo 
in  obedience  to  the  laws  of  the  inanimate  world,  accepting 
the  remark  of  Lamarck  applied  to  the  development  of  species, 
thst  "  time  is  nothing. "  The  only  supernatural  condition 
which  I  will  ask  the  reader  to  accept,  is  that  the  promontory 
on  which  we  keep  our  vigil  remains  unchanged. 

Looking  acro'^s  the  shimmering  sea  of  fancy,  we  sec  the 
new-born  consequeni^  streams  appearing  like  shining  threads 
of  siK'er  when  the  skies  arc  tlcar»  but  when  the  rain  descends 
In  torrents  and  the  soil  is  loo-scned  and  disturbed  they  be- 


THE  LIFE  HISTORY  OF  A   RIVER 


303 


the  clouds 

y  irregular, 
usand  feet, 
n  all  direc- 
in  hollows 
)n  filled,  or 
ns  of  their 
\^.  that  are 
have  their 
ore  prosaic 
they  carve 
ndings. 
It  of  view 
:e  offer  no 
confidence 

a  century 
he  illusion 
violated. 
3grapher  ? 
undergo 

accepting 
3f  species. 

condition 

)inontory 

c  sec  the 
threads 
ilcsccnds 
'Hoy  be- 


come yellow  \/ith  sediment.  Already  changes  are  in  pro- 
gress. The  streams  charged  with  silt  and  sand  are  corrading 
their  channels.  The  lines  thus  produced  are  delicate  at  first, 
but  soon  become  more  and  more  deeply  engraved.  These 
infant  streams  have  their  sources  not  at  the  summits  of  the 
island,  but  in  gene»al  midway  down  its  sides.  The  deep- 
ening of  the  channels  leading  from  the  higher  portions  of 
the  island  to  the  sea  makes  the  waters  flowing  down  them 
master  streams.  As  they  sink  deeper  and  deeper  into  the 
rocks,  lateral  streams  are  developed  on  the  original  inter- 
stream  areas.  These  branches  become  swifter  as  the  main 
streams  deepen  their  channels,  and  in  turn  develop  branches 
to  which  they  themselves  arc  masters.  The  secondary  and 
tertiary  branches  cannot  excavate  below  the  level  of  the 
stream  to  which  they  contribute  their  waters,  but  the  down- 
cutting  at  their  mouths  may  keep  pace  with  the  lowering  of 
the  receiving  channel.  This  process  of  throwing  out  new 
branches  and  the  growth  of  each  branch  by  terminal  bud- 
ding, as  it  were,  soon  lead  to  the  complete  drainage  of  the 
land.  Water  falling  on  a  \  portion  of  the  island  finds  a 
system  of  channels,  delicately  adjusted  in  size  in  accord  with 
the  part  they  have  to  play,  which  lead  it  back  to  the  sea. 

Our  i?!land,  we  will  assume  for  sini()licity,  is  composed  of 
nearly  horizontally  bedded  rock  of  various  degrees  of  hard- 
ness. The  influence  of  the  dip  of  the  beds  beneath  the 
original  surface,  discussed  in  a  previous  page  in  connection 
with  the  adjustment  of  subsequent  streams  and  the  develop- 
ment of  drainage  systems  under  the  conditions  there  de- 
scribed, need  not  be  repeated. 

The  headward  growth  of  the  feeding  rills  atid  broi/ks  of 


304 


RIVERS  OF  NORTH  AMERICA 


I 

t-K, 

»■ 

V 
I 


I 


the  main  consequent  streams  brings  them  into  rivalry  with 
each  other.  The  boundaries  between  opposite-flowing 
streams  in  the  central  portion  of  the  island  become  more 
and  more  sharply  defined,  and  the  positions  of  the  divides 
can  be  easily  traced.  From  these  divides  the  descent  into 
the  valleys  on  either  side  is  steep.  Hard  layers  in  the 
nearly  horizontal  beds  cause  many  cascades.  The  young, 
joyous  streams  fill  the  air  with  laughter.  The  cascades  came 
into  existence  low  down  the  course  of  the  streams  and 
gradually  retreated  toward  the  centre  of  the  uplift,  leaving 
shadowy  gorges  as  records  of  their  migrations.  It  is  only 
when  the  streams  in  their  lower  courses  have  deepened 
their  channels  nearly  to  sea-level  that  they  cease  to  be 
whitened  by  cataracts  and  rapids. 

As  we  watch  the  growth  of  the  streams  we  note  that  they 
deepen  their  channels  most  rapidly  not  at  their  mouths,  nor 
at  their  sources,  but  at  some  locality  between,  which  differs 
in  its  relative  position,  in  various  instances,  with  the  size  of 
the  stream.  The  rate  at  which  the  streams  we  are  observ- 
ing entrench  themselves  depends,  as  we  know,  on  their 
volume,  the  declivity  of  their  channels,  and  on  the  amount 
and  character  of  the  loads  they  carry.  It  is  the  resultant 
of  these  main  conditions,  rock  texture  being  essentially  the 
same  throughout  their  courses,  which  determines  at  what 
locality  conspicuous  changes  will  first  appear.  Near  their 
sources  the  grade  is  steep,  but  the  waters  are  divided,  flow- 
ing in  numerous  channels,  and  the  work  they  are  enabled  to 
accomplish  is  not  so  great  as  farther  down  the  slopes  where 
many  tributaries  have  united  their  energies.  We  need  not 
aga.in   ronskler  the  various   elements  of  a  stream  energy, 


THE  LIFE  HISTORY  OF  A    KIVER 


305 


'airy  with 
:e-flowing 
•me  more 
le  divides 
;cent  into 
rs  in  the 
le  young, 
ides  came 
jams  and 
t,  leaving 
It  is  only 
deepened 
ise  to  be 

that  they 
uths,  nor 
1  differs 
e  size  of 
observ- 
on  their 
amount 
esultant 
illy  the 
at  what 
ar  their 
1.  flow- 
bled  to 
s  where 
eed  not 
energy, 


but  from  the  fact  that  the  channels  through  which  they  flow 
become  conspicuously  deeper  midway  up  the  slopes  of  the 
island,  it  is  e"ident  the  most  rapid  corrasion  is  there  taking 
place. 

Below  the  locality  of  most  rapid  corrasion  the  slope  is  less 
precipitous,  and  although  the  volume  of  water  is  greater, 
the  rate  at  which  the  streams  corrade  decreases  all  the  way 
to  the  sea.  The  amount  of  rock  that  has  to  be  removed 
in  order  to  admit  of  the  sinking  of  the  channels  to  base- 
level,  however,  is  less  and  less  the  nearer  they  approach  the 
coast-line.  The  streams  near  their  mouths  are  thus  enabled 
to  reach  the  downward  limit  of  their  task  sooner  than  at 
any  locality  higher  up  their  courses,  in  spite  of  the  fact  that 
they  there  work  more  slowly  than  elsewhere,  except  perhaps 
at  their  extreme  head-waters.  Whatever  the  conditions,  it 
is  evident  that  any  portion  of  a  stream  at  a  distance  from 
its  mouth  cannot  be  lowered  to  baselevel  more  quickly  than 
the  portion  nearer  the  sea.  unless  possibly  by  solution,  as  the 
material  umoved  would  in  such  a  case  have  to  be  carried 
up  instead  of  down  a  gradient. 

As  we  watch  the  changes  in  progress,  we  note  that  after 
the  first  adjustment  to  inherited  conditions  is  made,  all  of 
the  material  removed  where  the  stream  beds  are  steep,  is 
not  carried  directly  to  the  sea.  At  first,  perhaps,  the  slopes 
were  such  that  the  debris  contributed  to  the  master  streams 
could  be  carried  all  the  way  to  their  mouths,  but  such  an 
adjustment  of  gradient  to  load  at  the  start  would  be  of  the 
nature  of  chance.  The  probabilities  of  a  stream's  inheriting 
a  gradient  perfectly  adapted  to  its  needs  are  almost  in- 
finitely   small.      Throughout    the   life   of   a   stream,    even 


3o6 


RIl   ''.RS  OF  NORTH  AMERICA 


though  external  conditions  remain  unchanged,  there  is  a  con- 
stant process  of  adjustment  of  gradient  to  suit  the  changing 
conditions  due  to  corrasion  and  sedimentation  in  various  por- 
tions of  its  channel,  and  also  to  variations  in  volume  and  load. 

This  process  of  adjusting  the  gradient  of  a  stream  channel 
in  its  several  parts  to  particular  conditions  of  volume  and 
load,  is  so  delicate  that  no  two  of  the  streams  we  are  watching 
will  carry  on  their  work  in  precisely  the  same  way.  In 
most  instances,  the  debris  removed  midway  down  the  course 
of  a  stream,  where  corrasion  is  most  active,  will  in  j)art  be 
deposited  lower  down  and  aggrading  begin.  Other  streams 
will  deepen  their  channels  at  their  mouths  to  baselevel,  and 
then  begin  to  broaden  their  valleys  and  spread  out  flood- 
plains.  As  the  streams  grow  older,  the  portions  of  their 
courses  where  corrasion  is  in  progress  will  slowly  recede  up 
stream  and  be  followed,  at  least  for  a  time,  by  an  extension 
in  the  same  direction  of  the  increasing  flood- plains. 

Clouds  gather  about  our  island  from  time  to  time,  and  it 
experiences  all  the  vicissitudes  of  climate  entailed  by  the 
position  it  occupies  on  the  earth's  surface.  Vegetation 
springs  into  existence,  and  the  land  is  clothed  with  grasses 
and  flowers,  or  deeply  shadowed  by  forests.  The  length  of 
our  vigil  is  so  great  that  possibly  the  character  of  the  flora 
undergoes  many  variations  owing  to  climatic  changes.  Al- 
though these  modifications  in  conditions  vary  the  lives  of 
the  streams,  they  do  not  stop  their  work. 

When  th»'  streams  have  deepened  their  channel  where 
they  approai  h  thu  Ht*i|  peiirly  or  quite  to  baselevel,  vertical 
corrasion  ceases  <ind  is  followed  by  aggrading,  while  lateral 
corrasion  continues. 


THE  LIFE   HISTORY  OF  A   RIVER 


307 


•e  IS  a  con- 
changing 
iriouspor- 
:  and  load. 
Ti  channel 
)lume  and 
;  watching 
way.     In 
the  course 
in  part  be 
er  streams 
ilevel,  and 
out  flood- 
is  of  their 
recede  up 
extension 
s. 

ne,  and  it 

ed  by  the 

egetation 

th  grasses 

length  of 

If  the  flora 

ges.     Al- 

(^  lives  of 

jcl  where 
ll,  vertical 
lile  lateral 


If  we  select  one  of  the  several  larger  consequent  streams 
for  special  study,  we  find  during  the  earlier  stages  of  its  life, 
that  debris  is  continually  being  supplied  by  its  swift  upper 
branches  in  excess  of  the  amount  the  sluggish  current  in  its 
main  trunk  can  carry  away.  In  consequence,  the  flood- 
plain  downstream,  from  the  localities  where  corrasion  is  in 
progress,  is  built  higher  and  higher.  During  high-water 
stages  accompanying  heavy  rains,  the  stream  meanders  at 
will  over  the  flat  bottom  it  has  given  to  its  valley,  and 
divides  into  many  branches.  The  position  of  the  stream  is 
unstable,  for  the  reason  that  abundant  deposition  of  debris 
raises  its  bottom  and  borders,  thus  elevating  it  above  the 
adjacent  areas.  When  floods  occur,  the  stream  breaks 
through  its  levees  and  chooses  a  new  channel,  which  is 
built  upon  as  before,  and  the  process  repeated. 

At  this  stage  in  its  history  the  stream  to  which  we  have 
directed  special  attention  will  have  many  high-grade 
branches,  in  which  corrasion  is  in  active  progress,  and  a 
low-grade  trunk  portion  where  debris  is  being  deposited. 
When  the  stream  is  corrading,  the  valleys  or  gorges  are 
steep-sided  and  present  V-shapcd  cross-profiles,  but  below 
the  region  of  waste  where  a  flood -plain  is  being  formed,  the 
valley  is  wide,  essentially  flat-bottomed,  and  has  flaring 
sides.  It  is  to  be  noted  also  that  in  the  alluvial-filled  valley 
there  are  no  terraces.  During  this  still  youthful  stage  the 
trunk  stream  has  many  curves,  and  divides  into  several 
branches  during  floods,  so  as  to  enclose  low,  sandy  islands. 
These  changes  in  the  position  of  the  main  channel  are 
irregular,  and  frequently  rapid. 

The  grading  up  of  the  main  valley  in   the  manner  just 


^.M:,,li 


I 

i. 


■■: 


308 


RIVERS  OF  NORTH  AMERICA 


noted,  was  necessitated  by  the  high  grade  of  its  tribu- 
taries. The  aim,  we  may  say,  was  to  make  an  approxima- 
tion to  the  easiest  attainable  pathway  for  the  debris  on  its 
journey  to  the  sea.  Theoretically,  such  a  pathway  would 
be  what  is  known  as  the  "  curve  of  quickest  descent."  But 
the  down-cutting  of  the  stream  channels  in  their  upper 
courses  continually  changes  the  conditions,  and  a  continual 
process  of  adjustment  in  the  lower  and  flatter  portions  of 
the  curve  is  thus  necessitated.  As  the  grade  of  the  tribu- 
tary streams  is  thus  reduced,  the  region  of  previous  aggrad- 
ing must  also  be  modified.  Hence  there  comes  a  time 
when  the  stream  in  its  trunk  portion  begins  to  excavate  a 
channel  through  its  previously  formed  flood -plain. 

In  this  stage  of  adjustment,  centuries  being  numbered  as 
hours,  we  see  the  river  writhing  in  its  course  through  its 
more  level  tract ;  now  cutting  away  the  rocks  on  one  side 
of  its  valley  and  then  swinging  bodily  across  its  flood-plain 
and  attacking  the  opposite  bluff.  Each  of  these  migrations 
is  accompanied  by  a  multitude  of  minor  contortions.  Its 
course  is  always  serpentine.  On  each  of  the  minor  bends 
we  note  that  the  immediate  river  bank  is  steepest  on  the 
concave  side  of  the  curve  made  by  the  stream,  while  on  the 
opposite  or  convex  side  the  bank  slopes  gently  upward  to 
the  level  of  the  flood-plain.  The  stream  is  plainly  at  work 
in  removing  material  from  the  concave,  and  making  additions 
to  the  convex,  side  of  each  curve.  It  soon  becomes  appar- 
ent that  the  stream  is  working  over  the  material  forming  at 
least  the  surface  portion  of  its  flood-plain.  With  each  dis- 
turbance of  the  detritus  previously  deposited,  it  is  carried 
farther  on  its  way  to  the  sea,  but  each  journey  is  short  for 


1  \i 


THE  LIFE  HISTORY  OF  A   RIVER 


309 


its  tribu- 
)proxima- 
aris  on  its 
ay  would 
It."  But 
eir  upper 
continual 
ortions  of 
the  tribu- 
is  aggrad- 
es a  time 
:xcavate  a 

inbered  as 

irough  its 

one  side 

ood-plain 

igrations 

ions.     Its 

lof  bends 

!st  on  the 

c  on  the 

)ward  to 

at  work 

additions 

es  appar- 

irming  at 

each  dis- 

is  carried 

short  for 


all  but  the  very  finest  material,  which  is  taken  in  suspen- 
sion and  may  be  borne  to  the  sea  with  but  short  rests  on 
the  bed  of  the  stream.  This  process,  as  we  know,  leads 
to  an  assorting  of  the  debris  of  which  the  flood-plain  is 
formed  :  the  coarsest  portions  are  dropped  first,  and  on  them 
finer  and  finer  sediment  is  laid  down,  the  last  addition  to 
the  plain  being  the  finest  of  all.  With  each  period  of  rest 
in  the  flood-plain,  the  debris  undergoes  chemical  changes, 
and  is  more  or  less  affected  by  frost  and  variations  of  tem- 
perature, which  softens  and  weakens  it  so  as  to  favour  more 
rapid  wear  when  next  it  is  removed. 

The  increase  in  the  curvature  of  the  minor  bends  of  the 
meandering  stream  leads  from  time  to  time  to  the  cutting 
through  of  the  neck  of  land  between  two  adjacent  curves, 
and  the  straightening  of  the  contorted  channel.  A  shorter 
course  is  thus  made  for  the  waters,  which  is  followed  by  re- 
adjustment of  grade  both  up  and  down  stream,  and  the 
former  abrupt  curve  is  left  as  a  bayou,  the  entrance  and  exit  to 
which  soon  become  closed  and  an  "  ox-bow  lake  "  is  formed. 

A  single  migration  of  the  river  across  its  flood  plain  re- 
quires thousands  of  years.  But  during  this  time  its  channel 
sinks  deeper  and  deeper  into  the  previously  deposited  debris, 
and  an  entire  migration  from  one  side  of  the  valley  to  the 
other  and  back  again  is  not  always  completed  before  a 
second  swing  is  begun.  The  portion  of  the  flood-plain  not 
worked  over  during  one  of  these  incomplete  migrations  re- 
mains as  a  terrace.  With  each  migration  a.  flood-plain  is 
spread  out,  and  wherever  a  flood-plain  formed  during  the 
preceding  migration  is  not  completely  worked  over,  a  terrace 
is  left  as  a  record  of  the  unfinished  task. 


3IO 


RIVERS  OF  NORTH  AMERICA 


I 


The  branches  of  the  river  have  now  cut  down  their  chan- 
nels so  as  to  have  a  comparatively  low  grade,  except  at  their 
extreme  head-waters,  and  the  trunk  of  the  drainage-tree  is 
contorted  and  borden^d  by  alluvial  terraces. 

While  the  changes  outlined  above  have  been  in  progress, 
the  fine  debris  carried  by  the  river  to  its  mouth  has  been 
deposited  so  as  to  form  a  low-grade  delta,  which  makes  an 
addition  to  the  land,  thereby  increasing  the  distance  to 
which  the  river  has  to  carry  its  load  before  it  can  finally  lay 
it  aside.  Even  the  depositing  of  debris  in  a  delta,  however, 
can  scarcely  terminate  the  influence  of  the  river  upon  it,  as 
the  surface  of  the  delta  is  but  a  continuation  of  the  flood- 
plain,  and  future  adjustments  to  ever-changing  conditions 
may  necessitate  its  removal  and  re-deposition  in  a  later 
seaward  extension  of  the  land. 

The  growth  of  the  delta,  by  increasing  the  length  of  the 
river,  necessitates  that  its  gradient  throughout  the  portion 
where  flood-plains  occur  should  be  raised  in  order  to  facili- 
tate the  transportation  of  fresh  debris  over  it.  A  continual 
check  is  thus  placed  on  the  process  of  down-cutting  in  the 
alluvial  filling  of  the  valley,  necessitated  by  the  constantly 
decreasing  grade  of  the  corrading  tributaries.  This  con- 
tinual re-adjustment  of  the  gradients  throughout  a  drainage 
system,  be  it  a  meadow  brook  or  the  Mississippi,  with  ever- 
changing  conditions  of  corrasion  and  sedimentation,  is  one 
of  thousands  of  illustrations  of  the  harmony  of  Nature.  It 
was  the  result  of  this  process  of  continual  adjustment  which 
forcibly  impressed  Hutton  and  Playfair,  nearly  a  century 
ago,  as  is  shown  by  the  passage  quoted  on  a  pref  ::ory  page 
of  this  book.  


THE  LIFE  HISTORY  OF  A    RIVER 


3" 


leir  chan- 
>t  at  their 
ge-tree  is 

progress, 
has  been 
makes  an 
stance  to 
inally  lay 
however, 
pon  it,  as 
the  flood- 
onditions 
n  a  later 

jth  of  the 
portion 
to  facili- 
continual 
ng  in  the 
instantly 
lis  con- 
drainage 
th  ever- 
n,  is  one 
ure.     It 
nt  which 
century 
ory  page 


During  the  centuries  that  have  passed  while  we  have  been 
considering  the  work  of  the  streams  flowing  from  the  island 
befor  us,  wonderful  changes  have  taken  place  in  the  topo- 
graphy of  its  surface.  The  sinking  of  the  stream  channels, 
but  partially  counteracted  by  aggrading,  has  left  ridges  be- 
tween  the  drainage  lines.  The  land  has  been  roughened  by 
the  cutting  of  valleys  and  canyons.  The  degree  of  this 
roughening  depends  mainly  on  the  altitude  of  the  land,  the 
stage  of  development  reached  by  the  streams  in  various  por- 
tions of  their  courses,  and  the  amount  the  land  has  been 
lowered  by  general  erosion.  The  streams  are  yet  young, 
but,  as  already  noted,  have  advanced  farthest  with  their 
task  of  removing  the  rocks  down  to  sea-level  in  their  sea- 
ward portions.  In  the  regions  of  low  relief,  near  the  sea, 
the  streams  have  already  passed  the  youthful  stage,  and  are 
subduing  the  landscape  not  only  by  lateral  corrasion  but  by 
deposition.  In  this  portion  of  the  island,  also,  vertical  cor- 
rasion having  ceased,  the  tendency  of  weathering  to  reduce 
the  interstrearn  areas  to  the  •(  vel  of  the  adjacent  valleys  is 
no  longer  counteracted.  In  the  higher  portions  of  the  island 
the  divides  between  tne  main  streams  and  between  neigh- 
bouring branches  of  the  same  trunk  drainage-line,  are  sharp- 
crested  ridges.  There  is  a  wonderful  and  beautiful  system 
displayed  by  these  ridges.  They  are  not  level-topped,  but 
marked  by  peaks  and  downward-curving  saddles.  On  each 
ridge,  whether  a  main  divide  or  the  crest  of  a  branching  spur, 
whjre  two  streams  head  against  each  other,  there  is  a  sag 
or  saddle  in  its  crest-line,  and,  where  lateral  spurs  or  sec- 
ondary or  tertiary  ridges  join  the  main  divides  or  a  second- 
ary ridge,  there  are  peaks  or  rounded  knobs.     The  upland 


•>%. 


IMAGE  EVALUATION 
TEST  TARGET  (MT-3) 


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1 1.25 


Ao    12.0 


2.2 


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312 


RIVERS  OF  NORTH  AMERICA 


with  its  multitude  of  crest-Hneg  and  many  peaks  resembles  a 
vast  tent  supported  by  many  poles.  The  place  of  each  sup- 
port is  marked  by  a  peak,  roundea  and  given  a  convex 
curvature  by  weathering,  and  between  the  peakb  the  tent- 
cloth  descends  in  gracefully  curving  folds. 

The  branches  of  the  stream  are  most  numerous  where  the 
original  slopes  were  steep ;  the  interstream  areas  are  there 
narrow  and  sharp  crested ;  farther  toward  the  sea,  where 
the  slopes  are  more  gentle,  the  interstream  areas  are  broader 
and  flatter.  There  are  two  principal  reasons  for  these  dif- 
ferences. On  the  steeper  slopes  the  run-off  is  greater  in 
proportion  to  the  total  rain-fall  than  on  lower  slopes,  because 
the  less  the  slope  the  longer  the  waters  are  retained  on  the 
surface  and  the  greater  the  loss  from  evaporation  and  per- 
colation. A  more  important  result,  perhaps,  is  that  on 
steep  slopes  corrasion  is  rapid  in  proportion  to  weathering 
and  general  degradation,  while  with  progressively  decreas- 
ing declivity  weathering  more  and  mor2  nearly  keeps  pate 
with  corrasion. 

The  ridges  between  adjacent  streams  not  only  lose  their 
even  crests  as  determined  by  the  original  slope  of  the  land, 
wi*:h  the  progress  of  stream  sculpturing,  but  if  we  look 
down  on  them  from  a  point  vertically  above,  it  is  apparent 
that  they  are  sinuous  lines.  The  eating  back  into  the  up- 
lands of  qpposite-flowing  streams  has  not  been  uniform,  but 
the  divides  have  been  pushed  one  way  or  another  according 
to  the  rate  at  which  the  rival  streams  have  been  enabled  to 
progress  with  their  work.  The  divides  migrate  toward  the 
weaker  streams.  The  ridges  forming  both  the  main  and 
secondary  divides  are  sharp  and  steep-sided  where  opposing 


ii|> 


esembles  a 
F  each  sup- 

a  convex 
b  the  tent- 
where  the 

are  there 
ea,  where 
re  broader 
these  dif- 
greater  in 
s,  because 
ed  on  the 
I  and  per- 
s  that  on 
eathering 
r  decreas- 
eeps  pacv'? 

lose  their 
the  land, 
we  look 
apparent 
0  the  up- 
orm,  but 
iccording 
lablcd  to 
ward  the 
nain  and 
Dpposing 


THE  LIFE  HISTORY  OF  A   RIVER  ,45 

Streams  have  eaten  back  the  farthest,  and  are  broader  and 
have  more  gentle  slopes  where  diverging  ridges  meet.  The 
hills  as  well  as  the  valleys  are  ever  changing  and  record 
the  workings  of  laws  which  produce  infinite  variety  with 
the  constant  preservation  of  harmony  and  beauty. 

At  a  more  advanced  stage  in  the  history  of  our  island,  the 
peaks  standing  at  the  junction  of  lateral  ridges  become 
niore  and  more  prominent  as  the  saddles  between  them  are 
deepened.  Those  well  down  the  general  slope  in  time  be- 
come  so  far  isolated  that  they  stand  as  individual  eminences, 
and  their  connection  with  the  central  system  is  at  the  first 
glance  not  apparent. 

The  island  has  now  reached  its  greatest  topographic  diver- 
sity.  and,  unless  the  orderly  progression  is  disturbed,  as  by 
renewed  elevation,  for  example,  future  changes  will  be  in 
tne  direction  of  subduing  its  relief  and  smoothing  out  its 
contours. 

The  process  of  broadening  the  valleys  in  their  lower  courses 
is  extended  farther  and  farther  toward  their  sources.  Broad 
flood-plains  in  time  reach  well  into  the  c-ntral  group  of  hills. 

In  the  uplands,  valley-deepening  continues,  but  the  ratio 
of  corrasion  to  general  erosion  becomes  less  and  less  with  de- 
crease  in  the  gradients  of  the  streams.  With  decrease  in  eleva- 
tion, general  erosion  or  degradation  also  diminishes,  and  both 
corrasion  and  degradation  cease  when  baselevcl  is  reached. 

As  the  gradient  of  the  streams  in  their  upper  courses 
diminishes,  the  loads  they  carry  to  lower  tracts  also  become 
less,  and  the  streams  are  enabled  to  cut  still  deeper  into 
their  previously  formed  flood-plains.  With  the  advance  of 
old  age  the  gradients  of  the  streams  become  less  and  less 


^^m 


3H 


RIVERS  OF  NORTH  AMERICA 


r 
\ 


throughout  their  lengths,  but  are  always  steeper  near  their 
sources  than  at  any  locality  farther  downstream.        ;  : ;  \ 

Our  island  now  consists  of  two  portions  in  reference  to 
topographic  development :  a  central  region  of  prominent 
peaks  and  ridges,  and  a  surrounding  baselevel  plain,  covered 
with  a  sheet  of  stream-deposited  debris. 

If  at  this  stage  a  comparatively  sudden  elevation  of  the 
whole  island  occurs,  which  carries  it  up,  we  will  assume,  one 
hundred  feet,  conspicuous  changes  will  follow.  The  gradi- 
ents  of  the  streams  at  their  mouths  will  be  increased.  Some 
of  them  may  plunge  into  the  sea  over  escarpments,  thus 
forming  cascades,  which  will  recede  up  stream,  leaving 
sharply  cut  ravines.  The  gradients  of  the  streams  will  be 
re-adjusted  to  meet  the  requirements  of  the  changed  condi- 
tions. Their  revived  energy  will  enable  them  to  corrade 
throughout  their  length  and  to  quickly  deepen  their  chan- 
nels, especially  in  the  previously  alluvial-filled  valleys.  This 
rapid  deepening  will  lead  to  the  abandoning  of  portions  of 
previously  occupied  flood-plains,  and  terraces  will  appear. 
The  streams  flowing  through  their  new  steep-sided  channels 
will  at  first  retain  the  positions  they  chanced  to  have  at  the 
time  of  the  uplift,  and  will  be  sinuous,  but  their  increased 
energy  will  tend  to  straighten  their  courses.  The  terraces 
left  by  the  sinking  of  the  ii.reams  will  be  broad  in  the  coastal 
r)lain  and  become  narrower  and  narrower  farther  inland. 
At  the  same  time  the  vertical  elevation  of  each  terrace  above 
the  adjacent  portion  of  the  new  stream  channel  will  decrease 
from  the  sea  margin  inland  to  where  the  stream  it  borders 
ceases  to  be  a  depositing  stream  and  the  tract  where  corra- 
sion  is  in  progress  is  reached. 


THE  LIFE  HISTOR  Y  OF  A   RIVER 


315 


near  their 

ference  to 
prominent 
1,  covered 

ion  of  the  \ 
sume,  one 
Lhe  gradi- 
d.  Some 
mts,  thus 
I,  leaving 
ns  will  be 
;ed  condi- 

0  corrade 
leir  chan- 
ys.  This 
artions  of 

1  appear, 
channels 

ive  at  the 
increased 

2  terraces 
le  coastal 
r  inland, 
ice  above 

decrease 
t  borders 
:re  corra- 


At  this  stage  in  the  history  of  the  island,  its  marginal 
tract  is  an  upraised  peneplain,  surrounding  a  group  of  hills, 
or  monadnocks. 

The  streams  again  broaden  their  valleys  in  their  lower 
courses,  the  broadening  terraces  are  removed,  and  this 
change  sweeps  inland  until  perhaps  all  records  of  the  once 
conspicuous  peneplain  disappear. 

While  we  are  pondering  on  the  changes  produced  by 
slowly  acting  forces  when  the  time  limit  is  ample,  our  island, 
in  obedience  to  unseen  causes  Jeep  within  the  earth,  is 
depressed  two  hundred  feet.     The  sea  encroaches  on  the 
land,  and,  extending  far  up  the  valleys,  converts  them  into 
estuaries.    The  highlands  between  the  valleys  become  capes, 
and  possibly  some  of  the  outer  members  of  the  central 
group  of  hills  are  entirely  surrounded  by  water  and  are 
transformed  into  islands.     The  coast-line,  which  previous  to 
the  subsidence,  was  conspicuously  regular  and  formed  long, 
sweeping  curves,  is  now  markedly  irregu:ar.     The  sea-water, 
extending  far  up  the  deeper  and  more  thoroughly  developed 
valley,  forms  long,  narrow  estuaries  or  downward  valleys, 
of  the  type  of  the  Hudson  estuary.     In  some  instances  the 
drowning  of  the  valleys  has  extended  above  where  the  lower 
branches  of  the  former  stream  joined  the  main  trunk,  and 
the  lower  courses  of  the  tributary  valleys  are  now  bays 
on  the  side  of  the  main  estuary.     These  are  estuaries  of 
the  Chesapeake   type.     In    such    instances   the   trunks   of 
the    former    river    systems    have    disappeared,    and    only 
their  dismembered   branches  remain.     The  drainage-trees 
have   been    betrunked    by   subsidence.     At   this  stage   of 
greatest  geographical  diversity,  so  far  as  the  relations  of  sea 


3i6 


RIVERS  OF  NORTH  AMERICA 


\\ 


r 
I 


i.  ■!  - 

(J 


and  land  are  involved,  the  shores  of  the  island  are  indented 
by  numerous  bays,  many  of  them  having  flat  alluvial  lands 
at  their  heads.  .  -  . 

In  fancy  we  have  clothed  our  island  with  a  varied  flora. 
The  picture  presents  a  pleasing  grouping  of  swelling  hills 
with  rounded  summits  and  gradually  sweeping  sides,  wide 
valleys  with  gently  sloping  borders,  separated  in  part  by 
broad  but  yet  well-drained  plains.  These  various  forms  are 
modulated  and  their  details  concealed  beneath  a  living  man- 
tle of  vegetation.  Seasonal  changes,  recurring  like  the  fig- 
ures in  a  dance  of  merry  children,  come  and  go  with  the 
ebb  and  flow  of  the  annual  tide  of  temperature.  Each 
springtime  the  willow-fringed  brooksides  blush  with  the 
pulsations  of  renewed  youth.  Flowery  banks  and  shadowy 
vistas  in  the  forests  reveal  cool  retreats  in  summer,  when  in 
the  stillness  of  the  evening  we  hear  the  distant  mellow  song 
of  the  wood-thrush.  The  deep,  strong,  harlequin  colours  of 
autumn  make  the  island  a  garden  of  gorgeous  flowers  edged 
about  by  the  silvery  surf.  In  winter  the  babble  of  the  brooks 
is  hushed  beneath  icy  coverings,  and  the  bare  trees  are  etch- 
ings on  the  white  pages  of  the  snow.  These  minor  nar- 
monies  are  interwoven  all  through  the  melody  of  the  ages. 
Like  the  white  fretwork  on  the  waves  of  the  sea,  they  ac- 
company the  greater  changes  wrought  by  unseen  agencies. 
We  are  overpowered  by  the  multitude  of  questions  suggested 
by  this  great  drama  of  Nature.  We  long  to  know  what  the 
end  may  be.  Why  all  this  beauty  and  variety  in  form  and 
colour  ?  Why  the  scented  breeze,  the  hum  of  ii^sects,  the 
songs  of  birds,  the  music  of  the  brooks,  the  coming  and 
going  of  hills  and  valleys,  and  the  thousands  of  other  evi- 


THE  LIFE  HISTORY  OF  A    RIVER 


317 


e  indented 
uvial  lands 

Tied  flora, 
filing  hills 
iides,  wide 
n  part  by 
!  forms  are 
ving  man- 
ke  the  fig- 
•  with  the 
re.     Each 

with  the 
1  shadowy 
r,  when  in 
silow  song 
colours  of 
^ers  edged 
he  brooks 
5  are  etch- 
linor  nar- 

the  ages. 
,  they  ac- 
agencics. 
suggested 
'  what  the 
form  and 
sects,  the 
ming  and 
Dthcr  evi- 


dences of  harmoniously  working  laws  ?     Was  the  elevation 
of  the  land  in  a  far  distant  time,  the  crumbling  and  decay  of 
the  rocks,  the  long  journeys  of  the  finer  fragments  down  the 
streams,  and  their  deposition  in  alluvial  lands,  but  to  form 
a  soil  in  which  violets  and  lilies  might  take  root  and  furnish 
nectar  for  the  bees  ?     We  trace  the  origin  and  development 
of  material  things  to  intangible  laws.     These  at  first  seem 
but  the  children  of  our  own  brains.     We  soon  learn,  how- 
ever, that  they  are  not  only  external  to  ourselves,  but  sway 
and  guide  us.     Man,  too,  gathers  honey  from  the  flowers. 
Such  a  mighty  vision  rises  before  the  mind  as  we  watch  our 
island  passing  through  its  orderly  transformations  or  glance 
upward  at  the  changing  constellations  above  it,   that  we 
pause,  fearing  to  go  farther,  lest  our  fancy  lead  us  astray. 
Our  studies   have  brought  us  to  the  threshold  of  a  vast 
temple:  to  explore  it  we  must  grope  our  way  at  first  and 
laboriously  gather  facts  to  guide  us  in  the  same  manner  as 
in  attempting  to  trace  the  life  history  of  a  river. 

We  are  recalled  from  dreamland  by  a  new  element  in  the 
scene  before  us.  A  canoe,  buoyant  and  graceful,  rounds  a 
distant  headland,  traverses  the  belt  of  dark  water  just  out- 
side the  beating  surf,  enters  one  of  the  sheltered  bays,  and 
touches  the  shore.  Dark  men  clad  in  skins  step  upon  the 
beach.  The  light  canoe  is  drawn  part  way  out  of  the  water. 
Soon  a  column  of  blue  smoke  rises  above  the  tree-tops, 
spreads  inland,  and  vanishes  in  the  steady  blow  of  the 
breeze  from  the  sea.  As  time  passes,  other  savages  come 
to  the  island.  Villages  are  built.  Fires  sweep  through  the 
forests  leaving  black  ruin  in  their  wakes.  The  soil  is 
stripped  of  its  natural  covering,  and  for  a  time  erosion  is 


3i8 


RIVERS   OF  NO  A  TH  AMERICA 


c 
c 

t 


111! 


accelerated.  Large  quantities  of  soil  and  other  rock  debris 
are  washed  down  from  the  hills  and  encumber  the  more 
level  lands  below,  destroying  for  a  time  their  fertility. 
Generations  of  savages  come  and  go,  until  a  change  of  as 
great  moment  to  them  as  was  their  coming  to  the  animals 
and  plants  of  the  island  takes  place. 

For  the  first  time  a  sail  breaks  the  even  sky-line  of  the 
sea.  A  Half-Moon  borne  proudly  on  by  gentle  breezes 
nears  the  island  and  enters  one  of  its  forest-fringed  harbours. 
The  changes  which  follow  the  coming  of  civilised  man  need 
not  be  dwelt  upon.  Chief  among  the  events  due  to  the 
greater  wants  of  civilised  than  of  savage  men,  is  the  removal 
of  the  forests.  The  land  is  cleared  of  its  trees  and  shrubs. 
Other  plants  which  grew  beneath  their  shelter  are  extermin- 
ated. Ploughing  greatly  facilitates  the  work  of  the  rills  and 
rivulets.  The  precious  layer  of  soil,  thin  at  best,  is  mere 
rapidly  removed  than  formerly,  and  the  sources  of  its  re- 
newal to  a  great  extent  destroyed.  In  time,  fields  become 
too  impoverished  to  repay  cultivation  while  virgin  lands  can 
still  be  had  on  neighbouring  shores  for  the  taking,  and  are 
abandoned.  Destruction  follows  apace.  Gullies  and  deep 
canyon-like  trenches  are  cut  in  the  sides  of  the  hills,  and 
even  greater  desolation  results  in  the  plains  below  than  fol- 
lowed the  wild-fire  started  by  the  Indians.  The  island 
loses  its  archaic  loveliness.  Its  flora  is  largely  laid  waste, 
or  supplanted  by  the  growth  of  seeds  brought  intentionally 
or  by  accident  from  other  lands.  The  native  birds  and 
animals  disappear,  or,  like  the  plants,  are  displaced  by  others 
and  in  part  alien,  species.  The  changes  are  so  profound  that 
,they  are  felt  not  only  throughout  the  fauna  and  flora,  but 


THE  LIFE  HISTORY  OF  A    RIVER 


3'9 


rock  debris 
•  the  more 
ir  fertility, 
lange  of  as 
he  animals 

-line  of  the 
tie  breezes 
d  harbours, 
d  man  need 
due  to  the 
:he  removal 
md  shrubs. 
e  extermin- 
he  rills  and 
;st;  is  mere 
of  its  re- 
ds become 
II  lands  can 
ig,  and  are 
5  and  deep 
'  hills,  and 
,v  than  fol- 
he  island 
aid  waste, 
entionally 
birds  and 
by  others 
ound  that 
flora,  but 


impress  themselves  on  the  topography  of  the  land.  No 
longer  a  source  of  immediate  gain,  the  island  is  neglected 
and  abandoned. 

The  rivers  with  their  increased  freight  due  to  the  debris 
washed  from  abandoned  fields,  progress  more  rapidly  with 
their  appointed  tasks  than  before  the  forests  were  removed. 
Deltas  are  formed  at  the  mouth  of  the  streams,  the  estuaries 
are  filled,  and  in  time  the  waters  of  the  sea  are  displaced, 
and  broad  grassy  plains  due  to  construction  make  their  ap- 
pearance. The  coast-line  again  becomes  a  series  of  sweep- 
ing curves.  Tens  of  thousands  of  years  elapse  before  the 
last  of  the  conspicuous  results  due  to  elevation  and  subsi- 
dence become  obliterated,  and  during  this  interval  marked 
changes  have  taken  place  in  the  group  of  monadnocks 
forming  the  central  highland  of  the  island.  Some  of  the 
original  consequent  streams  were  larger  or  had  shorter 
courses  to  the  sea  than  their  competitors,  and  were  enabled 
to  develop  more  rapidly.  The  divides  at  the  heads  of 
the  more  energetic  streams  receded  and  new  territory  is 
added  to  their  hydrographic  basins.  This  process  of  cap- 
ture and  diversion  leads  to  still  greater  diversity  in  the 
topography.  The  struggle  between  streams  for  the  posses- 
sion of  territory  in  progress  along  every  divide  is  not  unlike 
the  struggle  for  existence  and  the  survival  of  the  fittest  in 
the  animal  world.  The  streams  develop  in  accordance  with 
their  environment.  Those  most  favoured  capture  the 
waters  of  their  less  favoured  neighbours  and  wax  stronger 
at  the  expense  of  the  weak. 

In  its  old  age  our  island  loses  the  roughness  of  surface 
produced  by  stream  corrasion.     The  ridges  and  peaks  be- 


320 


RIVERS  OF  NORTH  AMERICA 


come  subdued  and  their  outlines  more  flowing,  and  the 
valleys  broader.  In  time  low  mounds  alone  remain  to  mark 
the  site  of  once  picturesque  peaks,  and  in  the  broad  valleys 
sluggish  streams  meandering  in  sweeping  curves  carry  off 
the  decreased  water  supply.  Even  the  hills,  after  a  pro- 
longed old  age,  disappear,  and  an  undulating  plain  but 
slightly  elevated  above  the  encircling  sea  remains.  A 
geographical  cycle  has  run  its  course.  The  resulting 
peneplain  has  even  less  diversity  than  the  surface  of  the 
new-born  island.  The  streams,  now  flowing  sluggishly  on 
account  of  the  lowering  of  their  gradients,  are  too  feeble  to 
carry  burdens,  and  run  clear,  but  their  chemical  activity  is 
undiminished,  and  they  still  bear  invisible  loads  in  solution. 
Chemical  degradation,  previously  of  minor  importance  in 
reference  to  the  work  of  mechanical  agencies,  now  becomes 
the  more  potent,  and  the  final  reduction  of  the  land  to  sea- 
level  is  secured  by  the  removal  of  its  material  in  solution. 

The  waves  and  currents  of  the  sea  have  been  active 
throughout  this  long  history  in  producing  changes  which, 
however,  are  beyond  the  limits  of  the  present  discussion. 

After  the  streams  ceased  to  bring  debris  to  the  shore, 
which,  we  may  presume,  either  in  part  or  wholly  counter- 
acted the  attacks  of  the  sea  on  the  land,  the  low  coastal 
plains  are  washed  away,  and  finally  the  waves  roll  over  the 
site  of  the  vanished  island. 


*; 


I,   and   the 
in  to  mark 
5ad  valleys 
s  carry  off 
ter  a  pro- 
plain   but 
nains.      A 
resulting 
ice  of  the 
ggishly  on 
3  feeble  to 
activity  is 
1  solution, 
ortance  in 
V  becomes 
nd  to  sea- 
olution. 
;en  active 
^es  which, 
Lussion. 
he  shore, 
'  counter- 
•w  coastal 
.  over  the 


INDEX 


i€olian  corrasion,  mention  of.  29 
Ages  of  terraces,  relative,  170,  171 
Aggrading,  explanation  of  the  term, 
98 

Alaska,  characteristics   of  the   rivers 

of,  284-289 
Alluvial  cones,  description   of.    loi- 

109 

—  fans,  reference  to,  loi 

—  rivers,  characteristics  of,  264,  265 
Alsec  River,  Alaska,  mention  of,  284 
Analyses  of  river-water,  average.  80 

—  table  of,  78  •  K  , 

Analysis  of  rain-water,  75 

Anchor  ice,  influence  of,  on  stream 
transportation,  25-28 

Anticlinals,  influence  of,  on  topo- 
graphy, 198 

Appalachian  Mountains,  stream  ad- 
justment in,  195-203 

—  rivers,  brief  account  of,  260,  261 
Arctic  drainage  slope  briefly  defined, 

256.  257 
Atlantic   drainage    slope    briefly   de- 
fined, 256 

Babb,  C.  C,  observations  by,  74 
Baselevel,  definition  of,  47 
Baselevelling,  discussion  of,  46-50 
Bear  River.  Wyoming,  analysis  of  the 

water  of,  78 
Beheaded  streams,  explanation  of  the 

term,  191 

Bering  drainage  slope  briefly  defined 

257 

Big  Bend  of  Columbia  River,  Wash- 
ington, mention  of.  280 


Big  Wills  Creek,  Alabama,  adjustment 
of,  208-213 

Bfschof ,  G. ,  cited  on  water  analyses,  79 

Blatchley,   W.  S.,  reference  to  writ- 
ings of,  96 

Bonneville,  Lake,  reference  to  deltas 
of,  125 

Bottom  loads  of  streams,  68-70 

—  terraces,  origin  and  nature  of,  166 
167 

Branner,  J.  C,  reference  to  work  of 

X.,   II  ' 

Breccia,  due  to  faulting,  mention  of,  4 

Calcium  carbonate,  solubility  of,  92 
Call,  R.  E.,  reference  to  writings  of 

94 
Campbell,  M.  R.,  reference  to  work 

of,  X. 

Canada,  characteristics  of  the   rivers 
of,  290-292 

Canadian  Geological  Survey,  reference 

to,  xi. 
Canyon  of  Snake  River,  Washington, 

reference  to,  160 

—  rivers,  characteristics  of,  271-275 
Caribbean   drainage  slope  briefly  de- 
fined, 257 

Carrollton,    Mississippi,   sediment   in 

the  Mississippi  at,  71,  72 
Cascade     Mountains,     reference     to 

waterfalls   of,   57,    61 

—  streams  of,  280 

Catskill  Mountains,  migration  of  di- 
vides on,  251-253 

Cephalonia,  Greece,  reference  to 
"sea-mills"  of,  95 


33X 


322 


INDEX 


ill 


f    ! 
»•    ' 
1 
t 


f 

E 


Chamberlin,  T.   C,  reference  to  the 

work  of,  X. 
Chandler,  C.  F.,  water  analyses  by, 

78 
Characteristics    of    American    rivers, 

254-299 
Chattanooga,  Tenn.,  geography  near, 

208-214 
Chelan,   Lake,  Washington,   terraces 

near,  182 
Chemical  degradation,  82-84 

—  denudation,  discussion  of,  80,  81 

—  disintegration,  discussion  of,  6-n 
Chesapeake  Bay,  map  of,  219 
Chipaway  River,  reference  to,  138 
Clarke,  F.  W.,  water  analyses  by,  78 
Climate,  influence  of,  on  streams,  140- 

142 
Climatic    changes,    influence   of,    on 
streams,  223-233 

—  on  terrace-making,  158-160 
Colorado    River,    characteristics    of, 

271-275  ,,    , 

—  reference  to,  133  *    . 

—  still  corrading,  45 

Columbia  River,  characteristics  of, 
278-282 

—  terraces  of,  180-182 
Corrasion,  discussion  of,  28-36 

—  lateral,  discussion  of,  34-36 

—  stream,  general   process    of,    142- 

145 
Corthell,  E.  L.,  reference  to  writings 

of,  132 
Crevasses,  origin  and  nature  of,  120 
Crosjy,  F.  W.,  reference  to  writings 

of.  95 

—  W.   O,,   reference  to  writings  of, 

95 
Croton  River,  New  York,  analysis  of 
the  water  of,  78 

—  materia]  carried  in  suspension  by, 

79 

Cumberland  River,  Tennessee,  analy- 
sis of  the  water  of,  78 

Current  terraces,  origin  and  nature  of, 
167-169 

Curves  made  by  streams,  36-3'} 

Danube   River,  data  concerning,  74, 

75 

—  material  carried  in  suspension  by, 

79 


Darton,  N.  II..  cited  on  drainage  of 
Catskill  Mountains,  251 

—  reference  to  the  work  of   x. 
Davis,  W.  M.,  cited  on  lakes,  122 

—  cited  on  peneplains,  48 

—  cited  on  stream  development,  187 

—  references  to  the  writings  of,  x.,  41, 

42,  214 

—  and  J.  W.  Wood,  cited  on  super- 

imposed drainage,  243 

Decay  of  rocks,  i-ii 

Deflection  of  streams  owing  to  the 
earth's  rotation,  3<)-43 

Degradation  of  the  land,  general  rate 
of,  81-84 

Delaware  River,  analysis  of  the  water 
of,  78 

Delta  of  the  Yukon  River,  brief  ac- 
count of,  288 

—  terraces,    origin    and    nature    of, 

167-169 
Deltas,  origin  and  structure  of,  123- 

142 
Deposition,   stream,    general   process 

of,  142-145 
Deposits  made  by  streams,  variations 

in  the,  136-142 
Development  of  streams,  63-66 
Diller,  J.  S.,  reference  to  the  work 

of,  X. 
Discharge  of  the  Mississippi,  267 
Disintegration  of  rocks,  r-ii 
Distributaries,     explanation     of     the 

term,   103 
Diverted  streams,  explanation  of  the 

term,  192 
Divides,  migration  of.  247-253 
Dodge,  R.  E.,  cited  on  terraces,  157, 

rsS 
Drainage  slopes  of    North    America 

briefly  defined,  256,  257 
Drew,  F. ,  reference  to  the  writings 

of,  lOI 
Drift-wood,    influence  of.    on   stream 

development,  240-244 
Drowned  rivers,  examples  of,  260 
Dunes,     influence  of,     on     streams, 

139 
Dutton,  C.  E.,  reference  to  exjlora- 
tions  by,  x. 

Earth's  rotation,  influence  of,  on 
streams,  39-43 


INDEX 


323 


n  drainage  of 
,  251 

of    X. 
lakes,  122 
+8 

lopnient,  187 
ings  of,  X,,  41, 

ted  on  super- 
US 

owing   to   the 

-43 

i,  general  rate 

is  of  the  water 
iver,  brief  ac- 
id nature  of, 
cture  of,  123- 
sneral  process 
ims,  variations 

5.  63-66 

;  to  the  work 

isippi,  267 

,  i-ii 

ation     of     the 

a  nation  of  the 

247-253 

1  terraces,  157, 

orth    America 

J,  257 

0  the   writings 

of,   on   stream 

244 

lies  of,  260 
on     streams, 

ice  to  exjlora- 
luence    of,    on 


Elevation,  effects  of,  on  stream  devel- 
opment, 215-217 

England,  rate  of  land  degradation  in, 
83 

Erosion,  baselevel  of,  discussed,  46- 
50 

—  general  discussion  of,  46-5 1 

"Fall  line"  of  the  Atlantic  coast, 
brief  account  of,  261 

Fault  breccia,  mention  of,  4 

Fergusson,  J.,  reference  to  the  writ- 
ings of,  37 

terrel,  W.,  cited  on  the  rotation  of 
the  earth,  41 

Flood-plains,  origin  and  nature  of, 
I 10-116 

Floods  in  rivers,  brief  account  of 
22q-233\ 

Fluctuations'of  streams,  discussion  of, 
229-233 

Forshey,  Professor,  observations  by, 
70 

Fraser  River,  British  Columbia,  char- 
acteristics of,  282-284  ! 

I 

Ganges  River,  data  concerning,  75 

Geikie,  A.,  references  to  the  writings 
of,  18,  74  ^ 

Genesee  River,  New  York,  analysis  of 
the  water  of,  78 

Geographical  cycles,  definition  of.  49 

Gilbert,  G.  K.,  cited  on  Niagara 
Falls,  59,  60 

—  explorations  by,  x. 

—  reference  to  writings   of,    31     42 

125 
Glacial  corrasion,  mention  of,  29 

—  meal,  contribution  of,  to  streams 

14 

—  terraces,  origin  and  nature  of,  r6q 

170 
Glaciated  lands,  rivers  of,  262,  263 
Glaciers,  influence  of,  on  stream  de- 
velopment, 234-236 
Grand  Coulee,  Washington,  mention 

of,  280 
Great    Basin,    climatic   condition   of 

178.  179 
—  drainage,  brief  account  of,  257 
Great  Falls,  Canada,  mention  of,  58 
Great  Lakes,  rivers  flowing  to,  292- 
300 


Great  Plateaus,  references  to.  44   45 
Green    River,    Kentucky,    references 

to,  88-9(j,  94 
Ground  ice,  influence  of,  on   stream 

transportation,  25-28 
Gulf  drainage   slope   briefly   defined 

257 

Hayes,  C.  W.,  cited  on  geography  cf 
Southern  Appalachians.  208-214 

—  references  to  the  worl-  of,  x     207 

208  ■ ' 

Hicks,  L.  K.,  cited  on  flood-plains. 
118  ' 

—  cited  on  profiles   of  streams,   146 
.150 

High  Plateaus,  reference  to,  45 
Hitchcock,  E.,  cited  on  delta  terraces 
167 

Hoang  Ho  River,  data  concerning 
75  ^' 

Holmes,  W.  H..  cited  on  Colorado 
River,  273 

Hosford,   E.    N.,   watgr  analvsis  by 

78 

Hovey,  H.  C,  references  to  the  writ- 
ings of,  94,  96 

Hudson  Bay  drainage  slope  briefly 
defined,  256 

Hudson  River,  New  York,  analysis  of 
the  water  of,  78 

—  brief  account  of,  260 

—  cited  as  an  example  of  a  drowned 

river,  218 

—  material  carried  in  suspension  by 

79 

Humboldt  River,  Nevada,  analysis  of 
the  water  of,  78 

Humphreys  and  Abbot,  cited  in  ref- 
erence to  the  Mississippi,  70,  73, 
253.  267  ;  cited  on  drift-wood.' 
243 

—  cited  on  Mississippi  delta,  132 
Hunt,  T.  S.,  water  analysis  by,  78 
Hydration,  influence  of,  on  rock  dis- 
integration, 6 


Ice,  influence  of,  on  stream  transport- 
ation, 22-28 
—  weight  of,  23 

Indian  Creek,  California,  reference  to 
III  ' 

Invisible  load  of  streams,  75-Si 


324 


INDEX 


'\ 


I 


I 


I'  I 


Irrawaddy  River,  data  wOnceming,  74 

James  River,  Virginia,  analysis  of  the 
water  of,  78 

Jones,  W.  J.,  water  analysis  by,  78 

Jordan  River,  Utah,  analysis  of  the 
..ater  of,  78 

Jukes-Browne,  A.  J.,  reference  to  the 
writings  of,  18 

Julian,  A.  A.,  referen':es  to  the  writ- 
ings of,  zi,  76 

Keyes,  C.  R.,  cited  on  the  meander- 

ings  of  streams,  113 
Kittatinny   peneplain,    brief    account 

of,  200,  206 
Kowak   River,    Alaska,    mention  of, 

286 

Lake  Pepin,  VVisconsiv  -  Minnesota, 
reference  to,  138 

—  St.  Clair,  delta  in,  133 

—  Tahoe,  California-Nevada,  analy- 

sis of  the  water  cf,  78 
Lakes,  climatic  changes  indicated  l)y, 

220 
Laurentian  'iasin,  rivers  of.  2C2-3&J 
Le  Conte,  J.,  references  to  the  writ- 
ings of,  18,  ig 
Levees,  natural,  origin  and  nature  of, 

1 16-123 
Life  history  of  a  river,  301-320 
Limestone,  solubility  of,  92 
Loads  of  streams,  how  obtained,  13-16 
Loew,  O.,  water  analysia  by,  78 
London,    England,   analysis  of  rain- 

'vaier  at.  75 
Lookout    Mountain,     Tennessee-Ala- 
bama, drainage  of,  208-214 
Los  Angeles  Kiver,  California,  analy- 
sis of  the  water  of,  78 
Lost  rivers,  reference  to,  226 
Lupion,  N.  T..  water  analysis  by,  78 
Luray   Cavern,    Virginia,    references 

to.  93,  iH 
Lyell,  C,  reference  to  the  writings 
of,  33 

Mackenzie  River,  delta  of,  133 
Maine,  coast  topography  of,  218 
Mammoth  Cave,  Kentucky,  brief  ac- 
count ot,  88,  89,  91,  94 


Marsh,  G.  P.,  reference  to  the  wntings 
of,  II 

Mason,  W.  P.,  references  to  the  writ- 
ings of,  75,  76 

Matapediac  River,  New  Brunswick, 
anchor  ice  in,  25-27 

Material  in  suspension,  measures  of, 

70-75 

Maumee  River,  Ohio,  analysis  of  the 
wate'  of,  78 

Maxwell,  W.,  reference  to  the  writ- 
ings of,  II 

McGee,  W.  j.,  reference  to  the  work 
of,  X. 

Meandering  streams,  discussion  of. 
36-38 

Mechanical  disintegration  of  rocks, 
2-6 

Merrill,  G.  P.,  cited  on  hydration,  6 

—  reft-rences  to  the  writings  of,  9,  11 
MigratifjtJ    of   di.ides,  discussion   of, 

247-253 

—  of  waterfalls,  discussion  of,  60-63 
Mississippi     River,    analysis    of    the 

water  o,'',  78 

—  characteristics  of,  265-271 

—  Commission,  reference  to  map  by, 

1 22 

—  data  concennng,  74,  75 

—  delta  of.  131 

—  influence  of  earth's  rotation  on,  42 
--  iraindation  of,  1IQ-121 

—  material  carried  in  suspension  by, 

7') 

—  rate    of   degradation   in    basin    of, 

83.84 

—  se-'iment  in  waters  of,  70-74 
Missouri  River,  an  aggrading  stream, 

43.  44 
Mohawk   River,  New  Vork.  analysis 

of  ihe  water  of.  7i< 
Monadnock.  definition  of,  49 

—  Mount,    New     Hampshire,    tjfer 

ence  to,  49 
Montmorenci  Falls,  Canada,  rrference 

to,  58 
Morrill,  P.,  cited  on  the  Mississippi, 

267 

—  reference  to  the  writings  of,   121, 

253 
Moses  Lrke,   Washington,  reference 

to,  139 
Moulins,  mention  of,  34 


^4 


:o  the  writings 

es  to  the  writ- 

ir   Brunswick, 

measures  of, 

ina)ysis  of  the 

e  to  the  writ- 

e  to  the  work 

discussion    of, 

ion   of    rocks, 

:  hydration,  6 
itings  of,  9,  II 
discussion   of, 

jion  of,  60-63 
ilysis    of    the 

)5-27> 

>ce  to  map  by, 

75 

rotation  on,  42 
21 
suspension  by, 

in    liasin   of, 

{.  70-74 
|radiin^  stream, 

Tork.  analysis 

Pf.  49 
l)shire,    refer 

|ida,  rrference 

Mississippi, 

[ings  of,   121, 

3n,  reference 


\ 


INDEX 


325 


Murray,  J.,  cited  on  water  analyses,   ' 

80 

Natural    Uridge,   Virginia,   reference 
to,  94 

—  levees,  origin  and  nature  of,  116- 

123 
Newberry,  J.  S.,  explorations  by,  x. 
New   Engknd   rivers,    characteristics 

of,  259,  260 
New    Orleans,    i-.ouisiana,    depth    of 

delta  deposits  at,  132 
Niagara  Falls,  profile  and  section  at, 

60 

—  reference  to,  62 

Niagara  River,  characteristics  of,  296- 

29 
Nile  Kiver,  data  concerning,  74,  79 
Nita  crevasse,  Louisiana,  an  account 

of  the,  121,  i'?2 

Ott''wa  River,  (Canada,  analysis  of  the 
waier  of,  78 

—  mention  of,  292  ' 

Pacific  drainage  slope,  brief  account 

of,  257 
I'assaic  River,  New  Jersey,  analysis  of 

the  water  of,  78 
I'eneplain,  definition  of,  48 
Peneplains,  ancient,  in  the  Appalach 

ians,  205-207 
Platte  River,  Nebraska,  an  aggrading 

stream,  43,  44 
Po  River,  Italy,  data  concerning,  74, 

75.         . 
Porcupine  River,  Alaska,  ice-work  on 

banks  of   iX 
Pot-holes,  oritjin  and  nature  of,  33,  34 
Potomac  River,   Virginia,  data  con- 
cerning, 74 

—  rate  of  degradaii  n  by,  82 
Powell,  J.  W.,  cited  «m  baselevel,  47 

—  cited   on    moisture    necessary   for 

vegetation,  237 

—  cited  on  rapids  in  Color-xdo  River, 

138 

—  reference  to  explorations  by,  x. 
Precipitation,  influence  of  variations 

in,  on  streams,  224-228 
Profiles  of  streams,  145-151 

Rain-water,  impurities  in,  75,  76 


Reade,  T.  M.,  cited  on  chemical  de- 
gradation, 82 

Red  River,  Louisiana,  lakes  on  ♦*<€ 
sides  of,  122 

Regolith,  meaning  of  the  term,  9 

Reve'sed  streams,  explanation  of, 
192 

Rhine  River,  material  carried  1..  ns- 
pension  by,  79 

Rhone   River,    data   concerning,    74, 

75.  79 

Rio  (jrande,  data  concerning,  74 

Rio  Grande  del  Norte,  analysis  of  the 
water  of,  78 

River  piracy,  discussion  of,  203-205 

Rocky  Miiuntains,  reference  to  water- 
falls of,  57 

Rotation  of  the  earth,  influence  of,  on 
streams,  39-43 

Roth,  Professor,  cited  in  water  analy- 
sis, 79 

Russell,  L  C,  cited  on  glaciers,  236 

—  cited  on  Lau.tntian  basin,  293 

—  cited  on  Laurentian  lakes,  2(>| 

—  cited  on  terraces  of  the  Columbia, 

180-182 

—  references   to  the  writings  of,   11, 

24,  123,  124,  134.  137,  174 

—  T.,  reference  t'i  the    writings   of, 

of,  2''3 

Sacramento  Kiver,  California,  analysis 
of  the  water  of,  78 

St.  Anthony  Falls,  Minnesota,  refer- 
ence to,  62 

St.  Clair  Lake,  delta  in,  133 

St.  Lawrence  drainage  slope  briefly 
described,  "56 

St.  Lawrence  River,  analysis  of  the 
water  of,  78 

—  character  of,  43 

—  submerged  portion  of,  218 
Salisbury,    K.    I).,    reference    to    'he 

writings  of,  x.,  170 
Schooley  peneplain,  reference  to,  205 
.Screes,  explanatim  of  the  term,  109 
Shaler,  N.  S.,  cited  on  caverns,  yo 
Shenandoah  peneploin,  brief  account 

of,  200 
Shoshone   Fa'ls,  Idaho,  refer,  ice  to, 

.>2 
Sierra   Nevada,    reference   to   water- 
falls cf,  57 


\ 


326 


INDEX 


r  • 


Sierra  Nevada  rivers,  characteristics 
of,  275-278 

Sink-holes,  reference  to,  93 

Snake  River,  Idaho- Washington, char- 
acteristics of,  279,  280 

Snickers  Gap,  Virginia,  character  and 
origin  of,  200,  201 

Sr":.Iiern  rivers,  characteristics  of, 
263.  264 

Stevenson,  D.,  reference  to  the  writ- 
ings of,  18 

Stickine  River,  Alaska-Canada,  men- 
tion of,  284 

Stream  conquest,  discussion  of,  203- 
205 

—  deposition,  discussion  of,  97-131 

—  development,   discussion   of,    184- 

195 

Subimposed  drainage,  term  suggested, 
246 

Subsequent  streams,  origin  a-id  nature 
of,   184,  185 

Sulisidence,  effecti  of,  on  stream  de- 
velopment, 217-221 

Superimposed  drainage,  explanation 
of,  244-246 

Synclinal  mountains  and  anticlinal 
valleys,  207-214 

Synclinals,  influence  of,  on  topo- 
graphy, 198 

Tahoe  Lake,  California-Nevada,  an- 
alysis of  the  water  of,  78 

Taku  River,  Alaska,  mention  of,  284 

Talus  slopes,  origin  and  nature  of, 
109,  no 

Tarr,  R.  S.,  cited  on  drowned  rivers, 
219 

—  citeci  on  young  valleys,  55 

—  reference  to  the  work  of,  xi. 
Teanaway    River,    Washington,    dam 

of  drift-wood  on,  243 
Tcnipcrnture,    influence   of   variation 

in,  on  streams,  228,  229 
Terraces,  origin  and  nature  of,  152- 

183 
Thames     River,    England,    material 

carried  in  suspension  by,  1<.\ 
Thompson,    \V,  Ci.,  cited   on  anchor 

ice,  25-27 
Tides  in  Columbia  River,  280 
Todd,  J.  li.,  cited  (Hi  the  Mississippi, 

270 


Transportation  by  streams,  discussion 
of,  14-28 

Trenton  Falls,  New  York,  reference 
to,  58 

Troy,  New  York,  impurities  in  rain- 
water at,  76 

Truckee  River,  Nevada,  analysis  of 
the  water  of,  78 

Tundra  of  Arctic  shores,  brief  account 
of,  133,  288 

Underground  streams,  84-96 
United  States  Geological  Survey,  re- 
ference to  work  of,  xi 
Uruguay  River,  data  concerning,  74 

Vegetation,  influence    of,   on   stream 

development,  236-244 
Visible  loads  of  streams,  67-75 
Volcanic    agencies,  influence    of,  on 

stream  development,  231-233 
Volcanic  dust,  contributed  to  streams, 

14 
Von  Hosen,  J.,  table  of  analyses  com- 
piled by,  78 

Wales,   rate  of  land  degradation  in, 

Walker  River,  Nevada,  analysis  of 
the  water  of,  78 

Walla  Walla  River,  Washington,  re- 
ference to,  137 

Waller,  E.,  water  analysis  by,  78 

Water,  weight  of,  23 

—  analyses,  table  of,  78 
Waterfalls,   nature    and    history    of, 

54-62 

Water-gaps,  origin  of,  199-205 

Watkins  Glen,  New  York,  reference 
to,  58 

Wheeler,  W.  H.,  reference  to  the 
writings  of,  73 

White  River,  VVashingson,  mention 
of,  287 

Wilbur,  E.  M..  ciied  on  tides  \\\  Co- 
luml)ia  River,  280 

Willis,  H.,  cited  on  stream  adjust- 
ment, 198 

—  reference  to  the  work  of,  xi. 

—  reference  to  the  writings  of,  200 
Wills  Creek,  Alabama,  adjustment  of, 

208-213 


INDEX 


ms,  discussion 
ark,  reference 
rities  in  rain- 
i,  analysis  of 
brief  account 


^-96 

tl  Survey,  re- 

xi 

icerning,  74 

F,   on   stream 
W 

lence    of,  on 

231-233 
:d  to  streams, 

inalyses  com- 


Wind-gaps,  explanation  of  the  term 

199-205 
Wurtz    H.,  water  analysis  by,  78 
Wyandotte    Cavern.   Indiana     refer- 
ence to,  93  '         " 


1^7 


Yazoo^  River,  Louisiana,  reference  ta. 

Young  valleys,  illustrations  of,  55 
Yukon  Rwer,  ice-work  on  the  banks 

—  drift-wood  on,  242 


gradation  in, 
analysis  of 
shington,  re- 
is  by,  78 


history    of, 

19-205 

"k,  reference 

ence   to   the 

on,  mention 

tides  iu  Co- 

eani    adjust- 

af,  xi. 
gs  of,  200 
Ijustment  of, 


1P'«" 


•if 


■I 


nHHHHHHH 


i 


» 1 


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Man  and  the  Higher  Apes.    By  Dr.  A.  Keith,  F.R.C.S. 
Heredity.     By  J.  Arthur  Thompson,  School  of  Medicine,  Edinburgh. 
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